In the smallest sense of the word, Facebook has expanded to include mobile phones today. Just visit m.facebook.com with your web-enabled phone to interact with the mobile-optimized site. I say "in the smallest sense of the word mobile" because it's not adding much to the features and offerings. On the other hand, groovr is a social network I see as being highly mobile.. almost more mobile-oriented than not.
Speaking of which, Groovr has a ton of new features coming out soon (some debuting later tonight). Notable features include the ability to upload videos from your cameraphone and a major expansion of privacy settings.