I think the argument is flawed to begin with... $/user is just a bad comparison. I am no investment banker but I'd think valuations are calculated via a combination of revenue + growth.
If we look at Facebook's estimated 2009 revenue, which is $550M reported by TechCrunch, the valuation is about 20x. It's a bit overvalued at this point but not unreasonable, especially since Facebook's project revenue almost doubled 2008's.