Which says pretty much nothing about Facebook's continuing health.
for him.
Even if he were confident that Facebook has a good chance of growing, say, 10% YoY for the next 20 years, with those smarts and his access he can put the money to work on the next facebook, or even less successful companies that will likely beat that 10% in aggregate.
Facebook is big enough that when they start bleeding users somewhere else they'll just buy them.
I also noticed majority of my friends (even some PhD etc) are pretty embarrassed about their postings from few years ago how they pretty much throw up on their facebook wall daily about even silliest stuff. They cyber-grew-up and now would not do that kind of stuff again.
As of Thiel's sell, even FB on their recent call warned stockholders that recent successes will be hard to duplicate. As more move into mobile use and there is less space for ads than on your desktop, the only option FB has is to stuck more ads between your feed, lowering the quality of experience altogether.