We studied launchrock and other such viral invite engines, but decided to build our own.
We built a simple landing page at http://fab.com which contains pretty imagery of the types of products that will be on our site, with only 1 simple sentence explaining what we'll do, and only 1 call to action: request an invite.
Once user requests an invite, we introduce them to our viral invite process for "priority access." Priority Access (first allowed in once we open the doors) is based on number of friends invited, PLUS we created 4 levels of priority access, rewarding people who invite more friends with actual cash credits towards future use of the site.
We built in simple shares for Twitter and Facebook, as well as gmail, ymail, AOL, hotmail, bulk invites.
We started off with our own employees' contacts and then purchased a small amount of highly targeted FB ads to people in our target demo.
We A/B tested several variants of the site, including placement of open boxes for users to enter email addresses manually vs. prompting them to invite from their address books and FB/twitter (address books far outperformed).
FYI. So far, more than 20k people have signed up pre-launch and of the thousands they've invited by email, 16k have yet to signup.
FYI 2. More than 2000 signups have invited more than 5 people each.
FYI 3. So far the top sources of new users are: 1. User-to-user email invites 2. Facebook Shares 3. Twitter tweets
FB and Twitter are just about even though in terms of resulting in signups.