How to start a social network?

[José Carlos]

I have this great idea about a social network with a specific theme. I am a web designer and developer and very well know how to do it. The only thing I don't know is how to have a good start. I mean, in something like a social network if a visitor arrives and finds it practically empty (with few users), he or she is gonna leave. Even if I invested in advertising, the users that click the ad are going to visit the site and leave as soon as they realize nobody else is using it.. So how do I start? Should I create some fake users and fake activity or something like that? How has everybody else done it?