I have to wonder about this from Mark Zuckerberg:

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.

I rather suspect that he'll have to move faster than that - I think companies like Facebook are going to have incredible difficulty finding additional rounds of funding, given the current financial situation. They'll have to start making money the old fashioned way - by earning it. Do they have enough cash on hand for three years at their current burn rate? If so, they have 3 years. If not? They have the time between now and the end of that burn rate.