What every great internet company has done is to figure out a way to make money that has to match to what they are doing on the site. I don't think social networks can be monetized in the same way that search did. But on both sites people find information valuable. I'm pretty sure that we will find an analogous business model. But we are experimenting already. One group is very focused on targeting; another part is focused on social recommendation from your friends. In three years from now we have to figure out what the optimum model is. But that is not our primary focus today.

-- Mark Zuckerberg (first read here)