Mastodon getting some attention is actually pretty good. It's one of the first times in a while where an online social media network is largely free from corporate control. The code is free and open, and there's no algorithm pushing ads and PR - just a chronological feed of people you follow. And it's definitely more hospitable to leftists than Twitter is, since leftists will always be able to host servers and moderate them.

Hopefully more leftist servers come about, but in the meantime, you can find any server that has a decent description and rules and join that. I recommend sorting by uptime and open registration.