The community is the best thing about LIvemocha. People are always willing to help you improve. You can submit recordings of yourself speaking or writing submissions and other people will comment on them. You can also talk live with text, audio or webcam and there are tools to help with translation.
Various interesting articles related to SM and FLL.
On the attractiveness of social media for language learning: a look at the state of the art
Facebook-ing and the Social Generation: A New Era of Language Learning
Language Learners' "Willingness to Communicate" through Livemocha.com
Online gaming as sociable media