Have you seen their revenue growth? I think you have some really interesting points, but at the end of the day, Marc Andeessen looked at the deal and presumably said, "This is an opportunity" (at that valuation). That holds a lot of water with me and he certainly doesn't have anything to prove.
If you truly think there is nothing to learn from Zynga other than "get on the radar of Turkmenbashi", I really think you should look again.
I am not at epistemic liberty to speculate as to the reasons for the other partners' involvement in the round.
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...
Social proof is a big thing for most buyers, and VCs are no exception. But social proof doesn't render them idiotic. And Andreessen isn't a giddy MBA herd animal, he's a guy who's built and exited with two massive startups.
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.]