alt Hacker NewsMastodon isn't trying to compete anyway, it's not a Product(tm) driven by desire for profits, so the people who use it don't care if it "wins" or becomes more popular than whatever other platform. We're all just hanging out socializing and stuff, sharing neat things and discussing things like communities do. It's not easy to accurately estimate active Mastodon/Fediverse users, but then again, I'm not sure anyone really cares either way? lol
I've never been a fan of the "Product driven by desire for profits" line because it simplifies too much into too little imo. The reason people like scale is that it brings a diverse set of interests. For better or for worse, most Mastodon servers I've seen have a pretty similar set of interests. They're really the same constellation of interests I've seen heavily represented on internet forums since the late '90s. There's nothing wrong with that, but between IRC, web blogs, web forums, Reddit, HN, Discords, and conventional social media, I don't think you'll find a shortage of folks interested in those topics.
Meanwhile there's a lot of topics where a critical mass of fans only really exists on Big Tech Social Media and maybe the occasional subject subreddit. The hope of an open platform at scale is to expose those niche interests to open infrastructure.