I think an "appetizer" is necessary to get the users love the site and make them sent invitations to friends.
In case of YouTube, that was a video-clip that I adored, in case of Xing.com it was being able to see the profile of a friend I knew, and in case of Linked-In, it was seeing the profile of a world-wide famous developer.
I'm sure that, the most important factor driving the growth and affecting those viral marketing cofactors is this thing that I call "appetizer"
Anybody else get an annoying dialog from this site complaining that your cookies are disabled?