As for the (distant) competition, there's
- Google+, with a reported 170 million users as of last May, Facebook's already integrated one of Google+'s innovative features (Circles, which allowed users to separate business from social contacts). For now, though, Google+ users indicate they like that services cleaner interface (fewer ads) and integration with other Google services.
- LinkedIn has a reported user base of 150 million, and has focused on building a social network for working professionals.
- Twitter has a reported 555 million users, and a quite different pedigree and focus. Technically a microblogging service, it's becoming the preferred service for quickly sharing content. (Sometimes too quickly). Users also like Twitter's better record of keeping private information private.
- Pinterest has about 11 million users, and also provides a different type of content sharing - providing its users with a sort of virtual bulletin board where they can post pictures and other things.
- So.ci is a new social media offering from Microsoft. With the announcement that Windows Office will integrate more social-oriented features, and their introduction of SharePoint collaboration software, this may be the outlet for Microsoft's social features.
Facebook also has major competition in some foreign markets from native-language-based social media services.
Sources - 5 Facebook Rivals Hot On Its Heels, the Brainyard
Facebook Reports $1.18 Billion Revenues, Mobile Progress, the Brainyard