[patrickk]

"Imagine that, within a year, a modest 25% of Facebook users use their browser. That would equate to about 200M users; all of which are using their search."

I'd be willing to bet that a significant percentage of Facebook's users don't know what a browser is. They fire up Internet Explorer and are blissfully unaware that there are alternatives. This has been my experience with non-techies.

Those who do know what a browser is are likely to have Chrome, Firefox or Opera installed and probably see no personal benefit in downloading yet another browser. They are also unlikely to want to have Facebook invading their privacy even more.