Did they advertise on google or newspaper to have their fisrt 1000 users? or People just randomly ran across twitter website then signed up and started twitting and inviting friends?
http://articles.businessinsider.com/2011-04-13/tech/29957143_1_jack-dorsey-twitter-podcasting page 3 explains it in some detail… growing from an internal tool to a handful of adopters up to it’s SXSW explosion.
I haven’t seen much detail on how they marketed to those early people or if it was simply the spreading circle effect [who do I know, who do they know, etc] but it’s a great question I’d love to see more answers on too.
It was originally a platform for podcasting company, Odeo, as a way for their employees to send SMS messages to one another. So, basically its first base of users are private groups until it was opened in public on July 2006.