In theory, Lemmy and Mastodon are compatible with one another, as they both use ActivityPub.

In practice:

Mastodon users can only see Lemmy posts as Boosts ("retweets"), and from what I hear, it's fairly annoying and not a good experience

Lemmy users can't see anything on Mastodon at all, Lemmy doesn't have a way to federate with Mastodon instances yet.

This is the first time Lemmy has seen this many users ever, so I'm confident both of these issues will be fixed sooner or later. When they are, you'll be able to see Lemmy posts on Mastodon as if they were posts ("tweets"), and you'll be able to see Mastodon posts on Lemmy as some kind of post (not sure if the format has been decided yet).

@pankkake @ubergeek77 I'll actually use this as a an opportunity to test something - I copied your comment's link into mastodon and am replying to it with my mastodon account. I can see the thread in mastodon, and in theory, this reply should show up properly in Lemmy too.

I would like to add to your picture yggdrasil and matrix based messengers, as this will help infrastructure to be more robust and expand

@ubergeek77 @pankkake you can comment and make lemmy posts from Mastodon and others. I'm on Friendica for example and made this comment from my Friendica profile.

It's all a little arbitrary. When you create a new service (like Lemmy, or Mastodon), you can have them link with anything, in any fashion you like. The defaults are mostly sensible.

For example, I've just made a mastodon post asking /r/casual a question. Once that synchronizes across, you'll see the topic over there.