Facebook has reached a striking if inevitable milestone -- its active user base is now a billion strong, up from 900 million in April just before its IPO -- and it's commemorating that achievement with a brand ad designed to both celebrate those who log on every day and to spread the Facebook gospel as part of the tougher task ahead: reaching the next billion users.

CEO Mark Zuckerberg announced the news early this morning with a post on his timeline that links to the 90-second spot that lives on a Facebook page. It was created out of Wieden & Kennedy, Portland, and directed by the Mexican film director Alejandro Gonzalez Inarritu (of "Amores Perros" and "Babel" fame). While it has the polish of a TV spot and seems readymade for a prime-time debut, Facebook's stated intention is to promote it using its own suite of ad products -- including sponsored stories and the log-out ad -- though the video will live on a public page that non-users can also see.

Facebook has a history of publicly recognizing significant growth milestones. When it hit 500 million monthly active users in July 2010, Mr. Zuckerberg recorded a filmed greeting expressing his thanks. But the celebratory message currently being served up might be deeply appreciated within the company during a time in its history when it's taken its share of hits in the news cycle. In a moment of candor, chief operating officer Sheryl Sandberg said in her remarks during Advertising Week in New York City this week that the IPO "didn't go as we expected" and "we were obviously surprised and disappointed."

Rebecca Van Dyck, who became Facebook's first head of consumer marketing in February, noted that the project with Wieden had been in the works before her arrival but hadn't been pegged at its inception to the billionth user, since no one knew when that milestone would come. Ms. Van Dyck had previously been global CMO at Levi's and before that was a top marketer at Apple, where she worked on the launches of the iPhone, iPad, iPod and iTunes. Wieden, meanwhile, is the agency responsible for much of Nike 's TV advertising, as well as the Procter & Gamble Olympics work that focused on moms.

While the spot will be live worldwide starting today (including a placement on Facebook's log-in page, where it sometimes plugs its own products, like its App Center), Facebook will use ads to promote it in 13 markets: the U.S., U.K., France, Germany, Italy, Spain, Mexico, Brazil, India, Philippines, Indonesia, Japan and Russia.

These are a mix of both mature and emerging markets for Facebook. For example, Facebook's penetration in the Russian market is 4.8%, according to SocialBakers, and Mr. Zuckerberg's visit to Moscow this week to meet with Prime Minister Dmitry Medvedev underscores the country's strategic importance. User growth has historically been a revenue driver for the company, and since markets like the U.S. and U.K. are saturated, tapping new ad markets like Japan is key to keep revenue growing for the short- and medium-term as the company looks to eventually crack the code on mobile monetization.

While Ms. Van Dyck emphasized that the ad is about sharing the company's values with people who are already logging onto Facebook daily, she acknowledged there's a message for people who haven't joined yet.

"We're creating this for our users, but we think it's a message that will be interesting and relevant to non-users as well," Ms. Van Dyck said. "We feel like we need to be respectful and introduce ourselves and to say 'This is what we believe in' and 'Come on board.'"