What resources do maintainers use to find out who is actually using their OSS? Are there any opt-in tools that reveal some more info about actual end users/companies?

Maybe something @feross's socket.dev handles indirectly?

✨ Founder + CEO Socket (socket.dev) • 🌲 Stanford lecturer (cs253.stanford.edu) • ❤️ Open source at WebTorrent + StandardJS • 🧙‍♂️ Mad scientist

There's two kinds of users: There's the ones who added it as a dependency for some reason and live happily ever after, and the ones who end up opening issues/PRs.

Github does an okay job showing dependants, but those are less important.