While it is true, there are also some factors which make people converge on some platforms. Factors beyond simply presence of other people. Like technical features.
The biggest technical feature that draws users is the Interface. Twitter didn’t gain popularity (or even the Bird icon and “tweet” moniker) until Tweetie, Twitteriffic and eventually Tweetbot came along.
Reddit was just a website for (no offence intended) “neckbearded basement dwelling incels” until RiF and Apollo made it more accessible.
Mastodon usage soared when 3rd Party Twitter apps were killed and once once again when Ivory was released.
I didn’t even know about Lemmy until I heard @christianselig@mastodon.social and @gruber@mastodon.social mentioned it on The Talk Show. I didn’t start using it regularly until I discovered wefwef and Memmy.
You’re not wrong about there being multiple factors, but I’d argue that this is often the least important factor. The technical features are easily replicated. (See: threads, stories, reels/tiktoks/shorts, etc.)
Network effects, on the other hand, have a stranglehold like no other.
You’re on Facebook because your family’s on there. You’re on Twitter because your favorite meme pages are on there. You’re on Instagram because the photographer you really like is on there. So on and so forth.
