Facebook will never grow revenue to 10x (matching Google) by continuing to sell display advertising on its own site.

However, it knows so much about users and keeps them logged in, so it definitely has potential to serve highly targeted or viral campaigns everywhere else on the web and mobile apps, a much better version of adsense.

Secondly, it is involving itself deeply in to user's habits and eventually may make a big play in to e-commerce, getting a cut out of every transaction made by the user (while frictionlessly sharing the information, of course). Credits is just in its infancy.

Finally, its learning so much about user's habits on the web that it can do a better job than Google at search, and make a killing on search ads.

If all this happens, $100B is a bargain.