One of the most clever integrations of social learning into mobile learning I have seen comes from Transparent Language's Byki Global for iPhone.
As with many other flashcard type language learning tools, there are screens with the word(s) plus audio, quizzes and stats, but they added a Twitter link. When pressing the Twitter icon at the bottom of the screen, the most recent tweets using that word or phrase are shown in that language. How cool is that?
If you have an iPod Touch or iPhone you can download this app free from the Apple App Store. Included are essential words and phrases in 15 languages including Chinese, Danish, Dutch, French, German, Irish, Italian, Japanese, Norwegian, Polish, Portuguese (Brazilian), Russian, Spanish, Swedish, and Tagalog.
For the serious language learner, there are also single language apps available for $7.99.