Mastodon has been active for years, and since the twitter implosion, it is just as lively there as twitter ever was. Just more polite. I got the impression that we left all the trolls back on the birdsite, so mastodon is much nicer in tone.
There are several sites that can help you find your twitter-friends on mastodon based on if they have their username in their profile. Also, make new friends from across the fediverse!
There are a number of companies, newspapers, clubs, government agencies from different countries already on there, and also active. (I mostly know of the german ones, where even regional government hopped on there. They're even running their own instance.)
There are about a million instances now, so I'd recommend joining not a big one, but one that is thematically close to you. You will still be able to reach all (most) of the others, but the local timeline will be interesting. :)