Foursquare is increasingly becoming popular with the site now attracting 15 million users, tripling its base since this time last year. Half of the users are in the U.S., while the other 50 percent are international, ClickZ News reported the news.

.Foursquare and Facebook Places are increasingly smartphone users’ check-in services of choice. The other geo-social sites like Loopt, Scvngr, and MyTown – appear to have excellent growth opportunities in the U.S. in 2012.

Facebook claims to reach 350 million users globally. Meanwhile Foursquare has roughly 7.5 million domestic users. Soon-to-be-defunct Gowalla has garnered nearly 2 million worldwide users.

Whatever the case, the lion’s share of smartphone users aged 13 or older in the U.S accounting for 78 percent of smartphone users have yet to download Foursquare, Loopt, Scvngr, MyTown, Gowalla, etc.

Whether or not Foursquare can continue to build scale and whether Loopt, Scvngr, and MyTown can gobble up enough adopters to survive will be among 2012′s more intriguing storylines. If they don’t convert more of the masses to a check-in mindset, they may go the way of Gowalla and Whrrl.

Facebook has acquired more technical talent for its California headquarters by buying Texas based Gowalla.

Facebook released the following statement on Monday morning, “We’re excited to confirm that Gowalla co-founders Josh Williams and Scott Raymond, along with other members of the Gowalla team, are moving to Facebook in January to join our design and engineering teams. In talking with the Gowalla team, we realized that we share many of the same goals: building great products that reach millions of people, making a big impact quickly, and creating new ways for people to connect and share what’s going on in their lives.”