I think your argument is that Zynga, essentially being a video game company like the other three you mentioned, can innovate itself out of the -ville space once people start getting bored out of it. While this is certainly possible, we haven't seen any hint of innovation from Zynga remotely at the level of Blizzard, etc. This makes their long term prospects questionable, and thus I would be wary of their IPO.

I also would be extremely wary of a company that is so reliant on a fickle third party for its very existence. Unless of course all this Zynga exuberance is at the prospect of a Facebook buyout...