I think the Sword & Sworcery is a great example of how to effectively implement social media posting into a game. In this case it was strictly twitter, but I think they owe a large portion of their success to this feature.
Instead of posting scores or progress in a traditional sense, all of the story in the game is broken into 140 character chunks, which the user has the option to share as they want. Other people seeing these random, but interesting phrases showing up in their friend's feeds investigated and checked out the game. It also gave it quite a lot of press and buzz that wasn't directly related to its gameplay.
It offered a way for people to share aspects of their experience without it feeling like an ad.
This article offers a decent summary of how they made it work for their game: http://geraldonascimento.net/sworcery-how-4-four-guys-managed-to-hijack-the-world/
For some more empirical results, the superbrothers team estimates over 200,000 tweets sporting their #sworcery hashtag http://www.swordandsworcery.com/news/2011/6/7/applause.html