This may seem like a very basic question, but basic is where I need the most assistance. For tech start ups and social applications, or anything for that matter. How do most companies usually get their initial set of users?

An example to clarify my question: say you start a dating site for a specific niche. How do you get a good amount of users registered on your site, so that when you launch, you don't have people that register, see no activity, and then leave immediately?

I apologize if this is common knowledge ahead of time.