I run PPC and direct marketing campaigns on behalf of some pretty big game developers and publishers.
We have tried Adwords, Twitter, Facebook and paid placements is game sites (IGN, Gamespot etc)
Facebook has a real drag factor as it used to spam people with invites to play Facebook games - which for years included rewards for players for recruiting other players via Facebook (Farmville invite anyone?) Some people now delete and even report as spam anything game related on their timeline on Facebook.
Twitter via Twitter Cards (must be twitter cards and must be timeline) has won in terms of CPC and conversion every single time - and not by a small amount.
Facebook might get traction if you already had a community built around the game already onboard - or at least the genre of the game.
Scritty