I’m not sure if you would classify this as a “social media platform”, but imagine a federated MMO. Each server could specify its own rules for things like XP and drops, allow or disallow mods among its playerbase, or even have custom items and quests - but in certain areas (ie “in town”) all of this stuff would be disabled so that players from multiple different servers could all interact. You could choose a server based on whether you like a high pop or a low pop experience, temporarily try other servers out by partying up with someone from it, major guilds could run their own servers with their own events and stuff, and so on. Admins would want to defederate from poorly-moderated servers, servers with cheaters or with mods/rules that they think disrupt the experience they’re after, or whatever other reason.