This isn't a precise way of thinking about things, but I really think the most important part of maintaining the success of a social network is being "cool", or rather, being the social network that cool people use.

I don't know the data, but my assumption is that facebook's user count isn't really continuing to grow among young people in developed countries. Like it or not, this demographic is usually what defines what it means to be cool. Would people in Indonesia and India join Facebook if it were mostly old people and people from their own countries? I doubt it - they would know what young people in other countries truly prefer, plus they would likely be better served by a local alternative. Would old people join Facebook if their children and younger relatives used something else? I doubt that too.

Facebook wanted to grab Snap, and did grab Instagram, so that it could maintain its monopoly on the social media that "cool" people use. All we need is more companies like Snap not willing to be bought out for Facebook to see a big hit in this market segment. I think that's the long term threat to Facebook