How to get 45,000 pre-launch signups in your 1st 20 days
1. Create scarcity. Require future members to request an invite.
2. Create levels of priority access. The more friends who join, the higher level of attainment. See for yourself at http://fab.com/priority-access.
3. Provide easy ways for users to share on Facebook, Twitter, email contacts, etc.
4. Target influencers on Facebook. We took out a very small ad buy on Facebook targeted to less than 100,000 people who we thought would be big influencers. They have driven the viral growth.
We're now adding 4k users per day. Do a Twitter search for Fab.com and see for yourself the activity.
Oh, and it helps to be in a sexy category that a lot of people are interested, where there is no current leading online player (in our case online design sales).