A large portion of revenues for social games/worlds (Zynga, Playdom, Second Life, IMVU, etc) are gotten from users so passionate about the game they pay directly for credits - that is as clean as it gets and a solid revenue stream even if you cut out the bottom-feeder lead gen offers...

According to another techcrunch article about a third of zynga's revenues are from these types of promotions.

That being said - not all of them are of the scam variety so while Zynga may take a slight hit they should be fine.

Noting as well that this is a better long term position considering the social networks themselves would likely start policing these moves at some point.