I was only ever really into Reddit, which Lemmy has handily replaced.

• I technically have an Instagram, but I scrubbed it and removed the app to stop me from doomscrolling and giving Meta ad revenue.
• Microblogging like Twitter or Tumblr never really appealed to me.
• I was born after 2000, so Facebook doesn't exist in my world.

I do use Discord, but mainly because all my friends and family are on there. I would love if a federated alternative like Matrix became the standard. Hopefully the EU's Digital Markets Act will help - if I understand it correctly, Discord and other platforms will be forced to allow seamless bridges (which currently will get you banned) under interoperability requirements.

I don't personally use it but pixelfed is an Instagram type of site. It's part of the fediverse as well.

https://pixelfed.org/

Lemmy fulfills a sort of reddit/more intimate internet forum thing. Reddit's real social good was fixing shitty search engine results by using Reddit as a keyword, and for centralizing a lot of hobbyist discussion. Lemmy and the lemmylikes will get there as the platform matures.

I'm loving pixelfed as a sort of Imgur substitute tbh. Quick and easy image hosting for when I post on here. Not crazy about Instagram as a social media platform so there's nothing I need pixelfed to do besides let me host medium-sized images, caption them and copy the link.