Mastodon is also a serious competitor to Twitter, and slowly siphoning off it's users. That along with this app isn't good news for Twitter, but then Musk himself wasn't good news for Twitter.
At this point Twitter is already living off of it's historical strength, many of it's most popular users have no incentive to leave yet because they have a large following. So in theory you can still consume on the platform, and personally I'm still doing this.
I haven't tried Threads, and likely won't. I can't imagine myself signing up for another Facebook product. Mastodon, on the other hand, is a genuinely great platform.
Unfortunately, Threads eclipsed the number of Mastodon users within a few hours. As of this morning, it was at 30 million, while Mastodon hasn’t reached 10 million, and the number of active users is actually falling. I think they’ve missed their chance.
That's not necessarily a bad thing. Mastodon is a not-for-profit, nobody actively using it wants it to be another Facebook or Twitter, they want it to continue to be a user-friendly app that's not driven by a profit motive.
I wouldn't expect Mastodon to compete with Threads in terms of sheer numbers, because Facebook is piggy-backing off of it's own user base, which is massive. Mastodon, on the other hand, has 10 million users with essentially no deliberate marketing. That's quite the achievement.
I'd expect Threads to see an influx of bored users who like laying on their couch clicking buttons, but whether or not it's actually successful is another thing entirely.