One user at a time.<p>I haven't built anything solo with 1 million users, but I'm one of the 600,000+ people who helped build this:<p><a href="http://drupal.org/home" rel="nofollow">http://drupal.org/home</a><p>...and it was done one user at a time, over a decade, nothing unethical, not much advertising beyond word of mouth. Lots of hacks, though!