Farmville and the like are popular because they are integrated into Facebook. The fact that you can play the games "within" facebook is why they're so appealing, to facebookers.
Zynga is profitable as a company due to their less than honest billing policy. Didn't they get into big trouble for the scams they were running: "sign up for free* video games!! *with $19.95/month subscription to our worthless product catalog". They get you hooked on the games, then slowly make more and more content pay-only. It's like adding more and more nicotine to cigarettes.
Anyway, with so many other, and better, online game networks out there, why would Google put money into Zynga and not Pop-Cap (a publisher) or Shockwave (a hoster)? Yahoo has hosted their Games section for years, and I don't hear them complaining. Pop-Cap has been publishing online games for years, and I don't see them faltering either. Zynga? Seriously?