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. :)

I find it very active, once you follow a few active accounts. People boost interesting stuff into your timeline, you can follow hashtags directly, and on smaller thematic instances the local timeline is a great place too, so there is plenty of ways to get a nice active feed.

Mastodon is very active after you start following enough people and hashtags to populate your feed. It's a bit rough to get started though: no algorithm means no content (or very random content in the local/federated feeds) until you build it up for yourself. But once you hit critical mass, I've found it a much nicer experience than I ever got on Twitter.

Mastodon is substantially more active than Lemmy thus far. It's a different type of medium to Lemmy, so what you want to use is up to you.

Personally I use both, but just like I was spending much more time on reddit than twitter, I'd like to spend more time on Lemmy than Mastodon. I have also found Lemmy a little more intuitive to use initially, so it might be worth spending a couple weeks getting used to how the fediverse works before jumping into Mastodon.

I've only been on mastodon for a couple of days, but yes it is active, and depending on what you're after it might we worth it. For me, it seems to be a nice mix of keeping up to date with things and people I already know, and at the same time stumble upon (related) things I did not yet knew about.

You will have to put in some effort to follow people/accounts/hashtags, but that's kinda fun. Just try it out!

Never used twitter, now use mastodon, just makes more sense to me. Find an instance where you like the content being posted locally, that's the key, otherwise I would imagine you kinda have to DIY

I really like Mastodon, so much so that I have several accounts for different purposes. You do need to invest a bit of time into following a good number of people who post stuff you like, though. I was pretty active on Twitter and now I'm pretty active on Mastodon and I like Mastodon better, but I had to spend a little time searching hashtags for people posting cool stuff.

One thing that I haven't seen anyone mention is moderation - your experience will be better with a well-moderated, well-run server, otherwise you'll have issues with spam and maybe worse. Generally a medium-sized server based around a subject of interest or a geographic location works well, where the admins are actively involved in the community.

Mastodon is good if you're after following specific people, rather than just general topics.

I've bounced off Twitter and then Mastodon several times because my use for social media is more for link aggregation and discussion, and I don't really necessarily care about who I'm having a discussion with, but rather that there's a good discussion about an interesting topic going on.

I know hashtags are a thing and you can follow them, but I've kinda found them hard to deal with on Mastodon because everyone puts a giant pile of hashtags on everything so you end up following certain tags and like, maybe 1/3rd of the things that show up in the feed are actually really related to everything that was tagged.

I do run an instance and find it's useful for certain things, but I very much prefer the Lemmy approach to content.

Mastodon is great, especially if you use a well made app to browse it, like Ice Cubes, Ivory or Mona.

I love mastodon!

I’ve just started using both and find them to be complimentary.

I know you're specifically asking about Mastodon but consider checking it firefish as well. It's got a lot of quality of life features that Mastodon still does not. That said the app situation is a lot rougher. Most Mastodon apps do work with firefish but every now and again something seems to break. If you're looking for an instance firefish.lgbt is pretty solid but a little on the small side. Of course everything federates so that should be a huge issue unless you're really into local.

I love Mastodon, I was an admin for years and now am a user on another instance and both have been very positive experiences!

I love mastadon and highly recommend it! I've had nothing but a great experience thus far

There are other servers too working with the same idea as Mastodon which are cheaper to run, such as Akkoma and Calckey. It is a very active network and I love to use both: Lemmy and Akkoma every day.

It's nice and very active, but you might want to look up a tutorial or something, because the UI isn't very intuitive for new users (it's good once you know how it works though)

I have an epic experience there

Yes, very active, there is the #introduction tag there where you can find people (and people find you).