There's no doubt access to a large audience helped get some publicity and users early on. But to be fair, Stack Overflow worked well and was useful on day one. We've seen startups basically fold on the first day when things don't go smoothly. The site crumbles under load. There's no content on a user-generated-content site. Maybe the thing flat out doesn't work as advertised. Some of them eventually overcome these problems only to be ridiculed for it over the next year. The Stack Overflow launch was handled incredibly well. I would attribute their success to much more than the size of their audience.