You are assuming it needs to be replaced at all. These services are vapid additions to your life and once they are gone I think people will realize they didn't need to exist in the first place.
There are alcoholics who have dozens of drinks a week and there are people who responsibly augment a night of socializing with a drink or two. Alcohol isn't fundamentally irredeemable because some people develop problems, and neither is social media.
* Gamers are either tech-savvy or always have friends who are.
* Gamers stick to their small communities and leaders. If you superstar and announce that you're leaving Facebook nobody will follow, but gamers always follow their group / clan / whatever leaders.
* Also there is at least few companies like Valve, Blizzard, Epic Games who hold huge part of the market and have extremely loyal customers. They could certainly manage to set competition for Discord if there will be opening.