However, my understanding is that this is the land of Paul Graham, who famously and correctly pointed out here [http://www.paulgraham.com/startupfunding.html] that VCs "are like high school girls: they're acutely aware of their position in the VC pecking order, and their interest in a company is a function of the interest other VCs show in it."
The intense, excruciating fear of not being "in" on some lucrative action involving other investors readily willing to put up $100m+ is a formidable motivating force for putting your chips in the game.
By the way, I am not arguing that Zynga did nothing right or is a wholly inappropriate direction for a VC round of some description. This is the Web Economy, of course; more preposterous things have happened than stupendous investments in something as legitimately "virus-like" as Mafia Wars or FarmVille. But it is $180m we are talking about here...
But to dig into your assumption that $180m is ridiculous... The investment doesn't matter-- the valuation does. So let's guess that they sold a third of their company for that $180m-- that's a $540m valuation. Is that unreasonable? They reportedly have a run rate of $50m. I'll bet it's twice that in a year. What sort of revenue multiple do you think is appropriate for a high growth tech company? What kind of revenue/growth rate would be appropriate for a $540m valuation?
I'm all for poking fun at absurd valuations for companies that don't even have a glimmer of an idea for revenue, but these guys are looking at $50m/yr and growing like a weed. What's not to like?
[edit: another poster said a run rate of $200m - If that's true, wow.]