I think Facebook is more dug in to the web experience than MySpace or Bebo were, so the comparison is a little bit off. The real issue with Facebook is not that they aren't super-significant right now, but what their real asset is. Their asset is member information and member loyalty. At the moment, their "customers" are those advertisers who they can target onto those members. The problem is that the more they try that, the more the members are going to resent it. So far, Facebook has coasted on a bubble of hype where the advertising focus was not nearly as intrusive, persistent and manipulative as it needs to be for it to make the kind of money to justify such a huge price. Members are going to resent that more and more. They are going to seek alternatives.
Comparison with Google is closer, as Google has done well, but in fact Google is under-performing in relation to its share capital and I can see the price going down there long-term as well.
Amazon, PayPal and Ebay are all similarly afflicted; despite the vast capital and surging sales and market share, all of these Big Web companies are struggling to turn the level of profit the markets demand.
There will probably be other threats as well; Amazon is already attracting some scrutiny for neo-monopolistic practises; Google has been under fire in many territories. Facebook is currently a global monopoly and I can't see the authorities leaving that untouched for ever as they try and carve up the advertising space for themselves.