There were several factors that contributed to YouTube becoming the #1 video sharing service on the web. But a lot of initial adoption was driven by the fact that it had pirated content hosted on it. If you wanted to watch the latest episode of Lost for free, YouTube was your best bet: no queued downloading through torrents, just stream it from the server.
Via Guillaume Decugis
How to solve the chicken and egg issues hat marketplace services have. Faking it before making it can work. And can actually prove necessary not only to get traction but to give the right culture or identity to a service. Of course this form of bootstrapping doesn't scale but we know it doesn't matter: http://sco.lt/6n1fhh