1. I already had issues with Reddit long before the API drama, due to spam, etc. However when the API drama dropped, I almost 100% switched to kbin, save for the rare occasion I found some useful info on Reddit, with the occasional tumbling into some other interesting threads.
2. I post more often to Mastodon, slowly leaving Twitter too, I just need to convince my fellow authors to do so, to give me even less reason to use it. I however decided to limit posting updates of my game engine to Twitter, and instead use the main account now dedicated for it ( @PixelPerfectEngine )
3. I haven't heard of Matrix, I'll look up it. I don't have any major gripes with Discord so far.
4. Peertube is fine and dandy, however it's even less adapted than other Fediverse platforms, all while YouTube would be mostly fine IF IT DIDN'T ACTIVELY TRIED TO SABOTAGE MY FIREFOX! I EVEN PAY FOR PREMIUM AND MEMBERSHIPS!

I don't really use any other save for following a bunch of Misskey accounts with my Mastodon. However, I was thinking on creating a federated game launcher/gaming-focused social network similar to Steam. Some parts of it would use ActivityPub (especially public stuff like achievements), others XMPP or something similar. Issues are the question of a copy protection system, and an anti-cheat system. Banning pirated copies of the game from official networks is probably easy even with open source tools, and likely not so controversial. Anything more than that would require more complicated setups, such as a lot of proprietary stuff, not to mention are very controversial, especially with gamers. Anti-cheat systems are also a though issue, likely being the responsibility of the developer, and also there' a lot of controversy with them with kernel-level anti-cheat systems.