Twitter has also seen a mass exodus in the last few months in favor of a more open platform. And there's a real difference about Facebook in that on Facebook, you connect with people you know, so it's important to be on the same platform as them. It's harder for someone to leave the platform unless most of their friends move as well. Unsurprisingly, Messenger and Instagram are also really efficient in that regard.

This is not so true on Reddit, where people interacting usually don't know one another. What matters is the community, and communities can be smaller.

Well, that's how I use those platforms anyway, so obviously take this with a grain of salt.