I'm on Signal ... somewhat. I like it in principle, but the usability is poor and my connections on it are people who are either already ideologically aligned with it or sympathetic to my reluctance to use the Meta stuff (so my wife and a couple of friends).

It makes it hard to share contact details of a mutual friend (I didn't explore very far, but it wasn't obvious how if it was possible) so that alone will limit how fast it can grow versus other more viral options.

It's also a pain to keep the desktop version in sync with the mobile version - if you don't regularly login to the desktop version over some threshold time period it will stop syncing and you'll lose all conversation history from the interim.

I'd like it to thrive, but I just can't see it happening as it stands at the moment.