[munificent]

I think your analysis is the right way to look at it. Given how hard it is to actually leave Facebook, what this statistic probably means is "3 million fewer 13-year-olds joined Facebook than the number of 17-year-old Facebook users last year.

It does show that Facebook's demographics are skewing older. But I believe it's because their population is aging, not that existing users are leaving.

[shalmanese]

Facebook only reports DAU and MAU stats for users so leaving is easy. It just means not logging in for a month.

[poopsintub]

How does twitter report usage? I bet 50% of their users don't log in month-after-month if they wanted to skew that data.