Some thoughts in no particular order:
1. Yes, Facebook will be here for a very long time. They've done an exceptionally good job of making the service as sticky as possible for users, with games, marketplace selling, and soon their Libra coin.
2. Facebook and Discord is not free. There's a false sense of free with those services, where it's literally costing you your user data and privacy.
3. Whether or not Facebook and Discord are easier to start a new community on, they're exceptionally bad for large communities with a lot of activity. There's no long form communication or organization on those platforms, which makes to hard to save and retain the best content.
The one overwhelming takeaway from this article is that anytime you rely on another platform, you're at it's mercy. The only way to protect yourself is to run an independent community. You lose a lot of the platform's benefits, but you control your own destiny.