[alastair]

It's interesting to note that 27% of Google's revenue comes from partner sites [1], aka Adsense on 3rd party sites, where the "purchasing intent" is not so clear.

I personally think it's unlikely that Facebook will not launch an Adsense competitor at some point.

Then there's Facebook credits with at least 200 million users [2], potential there to expand into off-site micro-payments, POS systems, etc.

Or "Premium Features": advanced analytics, promoted posts, photo storage, etc

The $100B valuation is reflective of the market's confidence in their potential, obviously. It's wise to be cautious, but I wouldn't write them off just yet.

[1] http://investor.google.com/earnings/2012/Q1_google_earnings.... [2] http://www.businessinsider.com/facebook-credits-2011-6