Actually the "fighting the last war” effect was the reason everyone passed on Google. Almost all the VCs (and other companies) thought the search war and been fought and won five years before. Rather than looking at Google from an unbiased viewpoint (that Google was the first to solve search), they saw it as the 17th competitor in a mature market.

I think the difference with Facebook is the network effect and that it is run by Zuckerberg. What ever failings Mark has a CEO he has one amazing strength which is a single minded focus to let no competitor to Facebook rise. As he has shown time and time again he will buy you out or outspend if you have any possibility of being a competitor.