I know https://todon.nl/ is a good leftist instance for mastodon. https://hachyderm.io/ is general but nice admin team; and https://wetdry.world/ is general purpose and has a really modified (and fun!) fork of mastodon, and I vaguely recall seeing a couple Marxists there.

That’s AFAIK for mastodon. I’m not sure about the rest since I only use lemmy, piefed and matrix as my socials anyway, and even that’s a stretch lol.

I’ve been trying Mastodon for as long as Lemmy but it’s not nearly as good due to the “who to follow” problem and the lack of ability to sort a topic by amount of interaction. E.g. can sub to say #Canada but within that, there’s no good way to surface highly faved or replied-to posts. The upcoming Starter Packs should help a lot because you’d be able to follow bunch people who tend to discuss some topic, which means you’d get the more active posts in your feed by means of those people intercting with it. It doesn’t replace the need for sorting algos but it addresses the problem partially. Perhaps it would be good enough. Circles (same concept) worked wonders in Google+.

For Mastodon, I’d recommend just joining any large instance that’s been around for a while. You can always move accounts if you decide on a different one later. Mastodon makes it pretty easy, and can automatically notify your followers if you hop to a different one. You can also run your own with masto.host for like 6 bucks a month. I’m on pixelfed.social, and been pretty happy with it.

I’d just try some random general instance on each service. Especially if you want to try them. You can still move later!

That’s just how the fediverse works. Your instance only know accounts from other instances by boosting or directly searching for them. Not seeing most accounts from newly federated instances is by design.