There will be some justification for the high valuations when Facebook enters the display advertising space. Facebook will use like button log data and user profile information for superior ad targeting. Their ad targeting might not be very good now, but they are going to figure it out because that's where the money is.
Have you ever run ads on Facebook in their current format? They're terrible. We work in a very niche industry that is easy to target via FB profiles but results are much worse than Google.
Google has been at that for so much longer, though. While Facebook can mine my profile, Google has years of AdSense data about the actual sites I visit and what I do there, in addition to profile info from several different portions of my Google account (assuming they use that, I don't actually know).
One of the reasons reasons that Facebook is implementing features for third party sites (like buttons, the new comment system) is that Facebook wants to collect the same type of data that Google collects with AdSense.
The profile data and the interactions between users on Facebook is much richer than the interactions and profiles on Google. Facebook will eventually derive more value from their data than Google does from theirs.
