Ask HN: How would you reach to your first 1000 users if you were quora/reddit

How does website like quora or reddit would get their first 1000 or 10k users. Quora at least have features, where friends invite other friends, so it may have ripple effect. However reddit even doesn't have that. So how and where will these guys advertise.

I have been told that a good way reach users is to first identify them, see what websites they visit and then reach out to them though those websites. However its unclear to me that how would you reach user through other websites. Would you write link to you websites in comments of other website? Wouldn't that generate negative PR. E.g. quora knows that their users go to ask.com or yahoo/google answers, how would quora attract user from them to their website.

I also have some startup idea where user users generate some content and other discuss it. However its extremely unclear to me that how I would reach my first few users.