On her social-media and PR blog, Commentz, Sarah Evans and her staff compile a lot of stats. Each quarter, she cherry-picks the most relevant for marketers to share with Ad Age .

1. eMarketer estimates there will be nearly 21 million Twitter users in the U.S. by the end of this year, and a sizable minority of those will use the service at least in part to follow brands. (eMarketer)

2. Forty percent of bloggers consider themselves professionals. (MediaBistro/State of the Blogosphere 2011)

3. There are now more than 800 million active Facebook users, with more than 200 million added in 2011. (Social Media Examiner)

4. B2C Facebook results are 30% above average on Sundays. (Convince & Convert)

5. Tweets last up to 67 times longer for users with higher Klout scores. (Mashable)

6. Nearly every large charity and university in America is on Facebook. Less than 60% of the Fortune 500 are. (Grow)

7. B2B marketers are spending millions of dollars annually on social-marketing programs, though nearly 30% are not tracking the impact of social-media programs on lead generation and sales. (TechJournal/Pardot)

8. Thirty-four percent of marketers have generated leads using Twitter, and 20% have closed deals using Twitter (AllTwitter)

9. Roughly two-thirds of social-media users say that staying in touch with current friends and family members is a major reason they use these sites, while half say that connecting with old friends they've lost touch with is a major reason behind their use of these technologies. (PEW Research)

10. The vast majority (95%) of the parents of 10-year-olds on Facebook were aware when their child signed up for the site, and 78% of those parents helped create the child's account [despite rules that prohibit children under 13 from joining the social-networking site]. (CNN Tech)

11. One in three respondents (33%) said that they would prioritize social-media freedom, device flexibility, and work mobility over salary in accepting a job offer. (GigaOm)

12. One in three texters would rather text than talk. (NYTimes: Bits)

13. Seventy-seven percent of consumers said they interact with brands on Facebook primarily through reading posts and updates from the brands [....] 17% of respondents said they interact with brands by sharing experiences and news stories with others about the brand, and only 13% of respondents said they post updates about brands that they like. (Mashable)

14. The average Facebook user has 130 friends and is connected to 80 pages, events and groups. (Social Media Examiner)

15. Seventy-three percent of people think employees overshare on social-media. (Marketing Pilgrim)

16. Forty-three percent of all online consumers are social media fans or followers. (HubSpot)

17. Netflix's price hike caused 805,000 paid subscribers to jump ship in the most-recent quarter. (Mashable)

18. Sixty-four percent of Americans stream mobile video at work. (Tubefilter)

19. The Mobile Marketing Association of Asia stated that out of the 6 billion people on the planet, 4.8 billion have a mobile phone while only 4.2 billion own a toothbrush. (60 Second Marketer)

20. According to ThreatMetrix survey of 722 active internet using consumers, 37% intend to make a purchase using their smartphone, nearly three times as many as those who plan to use their tablet. (Get Elastic)

21. A 2011 study by the National Restaurant Association confirms that consumers who use social media, including apps, Twitter, Facebook, Foursquare, UrbanSpoon and more, not only dine out more, but are more likely to become return customers. (ReadWriteWeb)

22 . Ninety-five percent of Facebook Wall posts are not answered by brands. (All Facebook)

43. Twenty percent of searches on Google each day have never been searched for before. (HubSpot)

24. Tablet owners tend to consume a greater variety and volume of news on their devices, and tablets' visual, interactive features encourage in-depth exploration, according to a joint study from Starcom MediaVest and the online division of the BBC. (Mashable)

25. Auto-posting to Facebook decreases likes and comments by 70%. (Inside Facebook)

26. LinkedIn has 64 million users in North America alone. (All Twitter)

27. Twitter updates that include verbs have a 2% higher shareability than the average tweet. (HubSpot)

28. Facebook, Twitter and YouTube are now considered cornerstones of most social-media strategies in larger companies. Ninety-four percent of respondents said Facebook is one of their top three social media platform priorities. Twitter was second with 77%, and YouTube trailed with 42%. (Search Engine Watch)

29. SEO still dominates for marketers, with both B2B (57%) and B2C (41%) businesses stating it makes the biggest impact on their lead generation goals. (AllTwitter)

30. Overall, 57% of comments about U.S. airlines on social media in the past year were negative. But American Airlines -- the world's fourth-largest airline -- stood out with only 12% of social-media opinions about the airline being positive. (The Realtime Report)

31. Fifty-six percent of consumers said they are more likely to recommend a brand to a friend after becoming a fan on Facebook. (Mashable)

32. Fifty-six percent of college students said that if they encountered a company that banned access to social media, they would either not accept a job offer or would join and find a way to circumvent corporate policy.  (GigaOm)

33. Johns Hopkins, Facebook's birthplace Harvard, and Notre Dame are the top schools for social media. (Boston.com)

34. Only 15% of the average local business's fans are in the city where the business is located. (WSJ)

35. More smartphone and tablet owners are researching products than purchasing them -- 80.8% compared to 41.4%, according to BIGresearch -- but attitudes vary quite a bit among different age groups. (eMarketer)

36 . When you're cruising around the internet, how much of your time is spent on a social network or blog? According to a new study published by Nielsen, those two categories eat up 23% of internet usage overall. This is double online gaming, which comes in at number two and after that , it takes 75 different categories to account for the remaining 35% of time spent. (Marketing Pilgrim)

37, When it comes to liking brands on Facebook, the reasons are varied, but for the most part, respondents said they "Like" a brand on Facebook because they are a customer (58%) or because they want to receive discounts and promotions (57%). (Mashable)

38. There are 245 million internet users in the US, according to Internet World Statistics. Nielsen estimates that social media sites and blogs reach 80% of all active U.S. internet users. (PR Week)

39. IT professionals see serious risks associated with enterprise social network use -- and only 29% say they have adequate protection. (InformationWeek: The BrainYard)

40. There are now 100 million active [Twitter] users -- users who log in at least once a month -- with half of those users signing in at least once a day. Fifty-five percent of them access Twitter via mobile; 40% actually don't Tweet but simply dip into their Timelines to keep tabs on what people are saying. (CNN/Fortune)

41. Fully 65% of adult internet users now say they use a social-networking site like MySpace, Facebook or LinkedIn, up from 61% one year ago. This marks the first time in Pew Internet surveys that 50% of all adults use social-networking sites. (Wired PR Works)

42. The mean half life of a link on Twitter is 2.8 hours, on Facebook it's 3.2 hours and via "direct" sources (like email or IM clients) it's 3.4 hours. So you can expect, on average, an extra 24 minutes of attention if you post on Facebook than if you post on Twitter. (bitly blog)

43. Social media is responsible for one-third the web traffic in Malaysia. (ReadWriteWeb)

44. There are more than 3.5 billion pieces of content (web links, news stories, blog posts, etc.) shared each week on Facebook. (HubSpot)

45. According to a survey of 1,897 senior executives conducted by Weber Shandwick in partnership with Forbes Insights, 84% of the execs believe their brand's sociability is not up to world-class standards. (Mashable)

46. Beyonce's pregnancy news at MTV 's VMAs birthed a new Twitter record of 8,868 Tweets per second. (TechCrunch)

47. Forty-four percent of companies track employees' social-media use in and out of the office (TheNextWeb).

48. What makes people retweet? 92% interesting content. 84% humor. 66% personal connection. 21% celebrity status. 32% offered incentive (tangible or virtual). 26% "Please RT!" (WhiteFireSEO)

49. Among college students and young professionals, 24% experience three to five interruptions in a given hour, while 84% get interrupted at least once while trying to complete a project. (GigaOm)

50. All but 7% of social-media campaigns used Facebook. (All Facebook)