How did Digg.com grew from 0 users to what it is today?

initially it was word-of-mouth and most importantly kevin rose presented digg on "the screen savers" which helped all of his fan base to join digg. But they obviously had more marketing tactics that i am unaware of

I agree, Kevin Rose's television following gave an advantage that most don't have.

His tv following had a large part in it, but then people spread it beacause it's a good thing. Any webpage/utility that is new, provides a new service, or solves a problem with old services, has a much better chance of taking off than something that's not really new. Digg.com was one of the first sites of that nature, it was something new and people wanted it.