It's a funnel you can optimize like any other. For example, Hotmail could have A/B tested the footer copy to see which resulted in the highest conversion rate.

It's a funnel you can optimize like any other. For example, Hotmail could have A/B tested the footer copy to see which resulted in the highest conversion rate. That would also have told them something about how their product should be positioned in the market (e.g., whether people care more about flexibility or security). The iPhone is hard because you don't have many hooks. Loopt tried to make it work and then blamed it on an errant "bug," but it was clear they were trying to get a viral loop running over SMS. http://getsatisfaction.com/loopt/topics/how_can_i_never_receive_another_loopt_sms_invitation_ever http://getsatisfaction.com/loopt/topics/how_can_i_never_rece... Really you can do this over any social network, where "social network" is defined in the broadest sense as a collection of people with message-carrying connections. This includes email, IM, SMS, Facebook, snail mail, etc. More specialized markets work, too. For example, Xfire spread this way through gaming circles, and Raptr is trying to do the same thing except over third-party IM networks like AIM and Yahoo!