Mastodon, but I’ve largely dropped it for Bluesky. I’m no purist about full decentralization, and I think it’s enough that the latter both has users, and isn’t currently awful.

I’ve used Bookwyrm a bit. I kinda like it. But it had some UX issues imo, and didn’t keep me hooked.

Also tried Pixelfed. It’s pretty slick, but I just can’t see the point. Like Instagram, seeing a feed of just pictures wasn’t all that interesting to me… though I probably just haven’t followed the right accounts.