Recently the Facebook-equivalent in my country introduced an API for apps similar to Facebook's and a friend created an app that is doing quite well in terms of

Recently the Facebook-equivalent in my country introduced an API for apps similar to Facebook's and a friend created an app that is doing quite well in terms of users. The trick was to ask the users "Do you think this app would interest your friends?" and if they answered "Yes" it mass-invites all the friends. I think it's against the rules of the site, but since it's good for the site too, the rule is not enforced. I recall that FB also started out by mass-mailing the Harvard mailing lists. I guess virality often is just spamming or tricking the user into spamming.