My suspicion is that Facebook revenues could be much higher than they are letting on. I say that as a non-Facebook user and a fairly neutral outside observer. If they were being coy because revenues were actually too good, it wouldn't be the first company to do so. Google was coy about its revenue in the 2002-3 timeframe, and it surprised a lot of people when it was later revealed just how much they were making.

Here is my theory: Facebook's traffic, measured in terms of users or page views is about even with Yahoo. Facebook has the added advantage that every single page view is a from a logged in user, who has provided Facebook with a fair amount of personal information, far, far more than what Yahoo has. Facebook has considerable information from the users' profiles as well as their various activity streams that Facebook tracks. This should enable Facebook to target its ads more precisely than Yahoo can, and therefore achieve at least as much revenue as Yahoo. From an advertiser perspective, it would seem like Facebook, with its vastly superior knowledge of user behavior, should be a better bet than old-school Yahoo. Of course, Facebook probably hasn't pushed its sales efforts as hard as mature Yahoo; still, I would expect Facebook to be doing a goodly fraction of Yahoo's revenue, which is running north of $6 billion per year.

If you buy my theory, long term, Facebook should overtake Yahoo in revenue; in fact a lot of Facebook revenue will likely come by stealing advertisers from Yahoo. So the company with the most to fear from Facebook is not Google but Yahoo.