How do/did social apps get their first users?

Usually some kind of "cheat"; I wrote the registration system for a voice chat application for the Brazil market circa summer 2000.

We sent out maybe 30,000 unsolicited emails to a list we bought and got about 10,000 registrations (amazing response rate!)

We tried another mailing list which was not so good (had some addresses without @'s) and got hardly any registrations despite sending around 250,000 emails.

That 10,000 grew organically to at least 300,000 in the time I was involved.