Actually, myspace was jumpstarted by the company spamming 3 million people encouraging them to join up, so in a way they were born with a large amount of users.
I agree with FilmGod, some companies don't care about SEO because they've reached their tipping point some other way, and don't need to spend resources and money splitting hairs for a bit more SEO traffic. If you are just starting out you may need that kickstart, but focus on building something people will find interesting, not search engines.