Besides Lemmy, I have a Mastodon account. I’m not very active, though. I’m also on BlueSky, but because most of the post where uninteresting to me I uninstalled the app months ago and hadn’t logged in since. And I’m exploring Pixelfed, but my experience hasn’t been so good.
Yes, that’s exactly it: the discoverability. I joined a small to medium server, and I thought that Pixelfed’s search would be like the one in Mastodon: search for a hashtag and get results from all of the other federated servers, but no. The search function doesn’t seem to work with hashtags, so subscribing to one is a pain.
And the available apps are not very good. And the official one hasn’t been released yet.