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.