Advertising, ofcourse, and I'm sure you know how much of a storm has been kicked up in the past when things like 'in text' advertising were being trialed. The same uproar will be seen with Facebook if they cross that line between between a website with a user friendly environment, and website that looks more like a revenue raising playground.
Ultimately the minds behind Facebook are far far more innovative than any of us can claim to be, so they may have ideas that none of us could even think of to grow the business.
All I know is that their userbase has risen in an exponential manner, and all exponential growths plateau to a relative level at some point. Their revenue is directly proportional to their userbase, so will that plateau as well?
