I'm not sure I would consider WoW (a game with a monthly subscription and a 20+ GB client) to be a direct competitor with Farmville (a free-to-play game with a browser-based client). I suspect that they appeal to very different audiences.
http://blog.raptr.com/2011/09/27/zynga-report/
Specifically, ~20% of WOW players also played a Zynga game, and ~30% of XBox players also played a Zynga game.