I have researched various social networks dedicated to learning languages. Interestingly they all have their unique twist. From embedding flash cards for your websites to writing and becoming official translators (like yahoo answers).
Here are the few good ones I found:
Byki for example lets your create your own flash cards, with voice and share them on websites and other social networks. VoxSwap and iTalki reminds me more of a MySpace for learning languages. But professional looking with interesting methodologies are LiveMocha and Babbel.
Pick your choice, how do you want to learn or teach a language: like a geek, like a teen or like a traveler.