How did a site like Stack Overflow drive adoption?

I'm looking to build a user-content driven site with a model that's most analogous to Stack Overflow. SO has a really strong SEO position now because they have so many pages with well-targeted content. My big question is what kind of strategies did they use to get from 0 to 1000 users where they were able to ride the SEO snowball to millions of users? I know a big part is that the founders had a pretty strong blog following, but that's a really tough one to replicate.