Ask HN: Good examples of social networks with a game mechanism?

[agaton]

Rewards, achievements and game mechanisms in social networks is a strong force to get people engaged. Foursquare and Gowalla are two great examples of this.

So, I'm currently looking for more examples of this and how you can make this type of mechanisms to work. Any thoughts or examples out there?

[pedalpete]

Of course, you're on a social network right now which uses game mechanisms.

My understanding is the entire x-box live is based on game mechanisms.