Well, the only other ones I really actually use besides Lemmy are Mastodon and my InsaneJournal account. Been totally slacking on that, though, so gonna give it to Mastodon.
I don't include things like Wattpad or yt because I spend most of my time not actually communicating with others on either platform.
To be clear, I do own accounts on many other platforms, including Mastodon, as a way to keep those handles under my hand. I simply don't use them and see little reason to start using them.
The only other one I even use is Tumblr. And only when Lemmy is extra slow and boring. It's harder to actually just talk to people on Tumblr, the way "replies" work there. I use SM more for the social part. The memes and content are just bonuses.
I could (hypothetically) post "DEATH TO [Insert Name of Billionaire Here]" on Briar Public Forums and not get censored. Becauae its peer to peer and there no "server", there no moderatoes, in fact, there is no moderation at all (which is why images are not allowed on public forums).
I have seen a dude spewing kremlin propaganda about the war in Ukraine, and others countering the propaganda, but still have yet to see any bigotry (hopefully I don't ever see any).
Mastodon with tag subscription is pretty awesome. On the advance web client you can even do multiple rows with different hashtag subscriptions so it's like a self aggregating Lemmy
Excluding porn, YouTube is around 90% of my internet use. However, I must give credit to Twitter for the most accurate recommendation algorithm that almost exclusively only serves me content I'm interested in seeing and has been doing so consistently for a decade.