This is super hard and there is no great answer. I've never met anyone who's had good luck with places like Betalist. (Though "Show HN" can work.)

Realistically you are going to be tracking down your first 100 users yourself manually. Figure out who among your friends is a good fit and bug them. Then ask all your other friends who they know who'd be a good fit. If they don't click, follow up and ask why. If they click but then don't come back, follow up and ask why. If they become an active user, you are about to become best friends, always talking about what they like and why.

You're going to feel like a mooch for a while -- like you're always asking your contacts for things and not giving back. This is normal.

Also, this thread is on the front page, so post a link. Quick -- an opportunity!