I don't know what MAU means but I do know that humans are terrible at predicting things, common sense has little to do with metrics and, yes, of course Facebook

I don't know what MAU means but I do know that humans are terrible at predicting things, common sense has little to do with metrics and, yes, of course Facebook's job is to make money. The assumption that people leave assumes that there is a different place to go to, which for most people, there isn't because social networks are exactly that: networks. Unless you have a small isolated network that can move all at the same time, it is not likely that people will single themselves out.