Yes, Facebook's current network is fairly strong, but you shouldn't look to where the masses currently are, you should look to where the trendsetters, the young, and the people in the know—where they are. These people are ahead of the curve, and the signs they are emitting is that Facebook is not serving their interests and needs. Young people don't want to use facebook because it forces you to interact with everyone, including parents and people out of your circle. Privacy conscious people and many at the forefront of technology all either personally avoid and limit facebook usage or keep their children away from it (if they aren't already disinterested in the first place). Yes Facebook allows you to "fix" these issues with enough configurations, but then why would a new user go through the hassle of all that when they could just install snapchat or instagram (yes I know facebook owns this, but it's not facebook).

So unless there's a fundamental shift in its business model and allure, its trajectory will be doomed. It doesn't mean Facebook the company will fail, but it does mean that the Facebook product—what it is in its current form—has a finite shelf life.