Social logins remain an easy way for brands and publishers to get people to register to use their sites and apps in order to collect more information about those people -- who opt-in to share that information -- and do things like customize the articles or products they're shown.

Not surprisingly, Facebook remains the most popular platform that people use when logging in to brands' and publishers' sites and apps. The social network closed 2015 with 62% of the social logins tracked by Gigya, a company that provides tools for brands and publishers to let people log into their sites and apps using their Facebook, Google, Twitter, Yahoo or other accounts. Google followed Facebook with a 24% share of the market, trailed by Twitter at 7% and Yahoo at 4%.

To get a view into the social login scene, Gigya processed 58 million social logins across 700 brands and publishers' properties during the fourth quarter of 2015. And before delving into each company's respective share of the social login scene, it's worth looking at how often each of the four platforms is offered as a social login option. Again, not surprisingly, Facebook and Google are offered as options nearly 100% of the time. Twitter and Yahoo aren't. Twitter is offered 72% of the time and Yahoo 35% of the time.

In addition to the sheer size of Facebook's user base, Facebook has benefitted from the privacy controls it presents people when they log into a third-party site or app using their Facebook account, said Reeyaz Hamirani, Gigya's head of corporate communications who collected and analyzed the data that was provided to Ad Age. For example, people can see what information from their Facebook profile the brand or publisher wants to access.

Facebook has added to its social login lead specifically on mobile where it accounted for 80% social logins in the fourth quarter, compared to Google's 14% and Twitter's 5%, Mr. Hamirani said. Yahoo registered less than a percentage point.

While Facebook's dominance shouldn't be surprising, Google's relative resurgence of late may be. During the second half of 2015, the search giant shaved four percentage points off Facebook's share of total social logins. Facebook still ended the year with more than double Google's share of the market, but the third and fourth quarters marked the first time that Facebook experienced share declines in two consecutive quarters since Gigya began tracking social logins in 2011, Mr. Hamirani said.

"Both of the drops were attributed straight to Google," Mr. Hamirani said. "It's one of those things where Facebook may have hit a saturation point with social authentication." Additionally Google has tweaked the appearance of its social login buttons from showing the Google+ logo to now showing the Google logo, and that may have eliminated confusion among people who may have Gmail accounts but don't use Google+ and led them to opt for a non-Google login, he said.

The trend suggests that Google's introduction of its own social network Google+ in 2011 wasn't all for naught. Google+ may have been considered a Facebook wannabe -- and it never grew into a legitimate Facebook rival -- but a lot of people have maintained that Google+ was really about organizing Google's broad product line around a single universal account. The idea of a universal account is important because Facebook has been able to parlay its own user accounts into a digital advertising powerhouse. It is using those accounts to gather an increasing amount of data about its users, target them with ads on and off Facebook, and gauge how those ads translate into business outcomes, like sales and site visits, for advertisers.

Unlike Facebook and Google, the trend in Twitter's share of the social login market over the past year has stayed relatively flat, closing 2015 at 7%. That aligns with the slower growth of the social network's own logged-in audience, and Mr. Hamirani attributed it to the lower amount of information that Twitter is able to share with a brand or publisher, which limits those companies' abilities to personalize their site or app to the individual user.

In contrast to Facebook's dominance, Google's growth and Twitter's steadiness, Yahoo continues to see its popularity decline, which appears to reinforce media buyers' assertion that Yahoo's lack of a large enough logged-in user base has hurt its advertising business. Yahoo's share of the social login market peaked at 18% in the third quarter of 2013 and has dwindled to 4% in the fourth quarter of 2015, according to Gigya.