A really great post by Mike Turner from Bitfold Online Games.
The top companies in the social market have launched games which have millions of users and million in monthly revenue. However, most of the successful companies in the space are either totally new companies like Zynga or casual gaming companies like Popcap games or King.com. Only a small handful of developers & publishers established on other gaming platforms (consoles, MMO, etc.) have had any success in the social market.