Bejeweled, or the ever-growing MMO format.
It's a relatively new and growing market, and several developers sat down earlier today to take a look at it during the Flash Games Summit. Moderator Sana Choudray of Traffichoney led a discussion of what is needed for a successful social game, as well as where they stand now and where they will go in the future.
While the developers acknowledged that social gaming is lacking at the moment, they all agreed that it's only going to grow over the next year or two. The full discussion is well worth a read.