Around 2004, when tech writers were trying to explain why Friendster failed, and why MySpace was becoming such a power, there was a lot of writing about Metcalfe's law -- the more people who joined, the more valuable the network was. Then around 2009, when MySpace was dying, there were several articles that pointed out that the opposite was also true: the more that people withdraw from a network, the more that network loses value.
Perhaps we will see similar articles soon, making the point about Facebook. But I'm not expecting to see people abandon Facebook suddenly, the way people suddenly abandoned MySpace. Rather, the process this time seems a bit more subtle -- people realize that everyone they know (plus future employers) is watching, therefore they have to censor themselves, therefore they have to limit their engagement. The effect is the same, Metcalfe's Law in reverse, the network loses value.