Just learned about #pixelfed! Awesome, this occupies a badly needed decentralized space, for an instagram alternative.
Still needed: something that does whatever stupid stuff snapchat does (be weirdly different and hip I guess), and a decentralized reddit.
Honestly, I think you can already do a decentralized reddit by treating hashtags as subreddits, favorites as votes, and replies as comments. And whatever reddit you see depends on who you federate with.