I once started a social network around car culture. It did OK, and we got an exit offer, but decided to pivot to something else (which was the right move).

Anyways, the way we got our early users was nontraditional. We went in person to car shows, took pictures of the cars, making sure one of them was with a polaroid camera. We then handed the Polaroid to the owner of the car and asked if we could feature their car on our website. If they said yes, we wrote out a email/pass on the back of the polaroid and then went home and set up their account. This seeded our original social graph.

Harder to do these days, but don't forget about your legs. It's a lot easier to convince people in person than over a rote email.

We got on Techcrunch as well, but it didn't help nearly as much. Techcrunch is mostly good for the inner industry[0]

[0]: https://techcrunch.com/2006/06/11/cnet-veterans-launch-boomp...