Many web apps rely fully on viral spread. In order for this to work, a virality coefficient higher than 1 is needed; meaning one user invites more than one other user into the service. Lots of well known bloggers and authors, like Eric Ries, state that you should postpone your full launch and iterate until you have reached a virality coefficient above 1. I believed this for a long time and still think it makes perfect sense in many situations. However, I think there are exceptions.
The usual process to create a viral app used to be to start with a quiet launch to be able to optimize the viral loop and doing a full launch when reaching something above 1. I think optimizing the viral loop this way is still a must. But it is very hard to emulate real conditions. There are many variables that will have an impact. Some can be emulated, others are harder. For example, the conversion ratio of invitations to signups will most certainly be affected by how well known the brand is. If a user gets an invitation to a well known app like Angry Birds, the user will of course be more likely to sign up and try it out, than any previously unknown app.
My hypothesis is that there are many situations, where a virality coefficient of less than one during tests will be sufficient in the real world. The reason is that it may be possible to improve it with some big PR event, where you influence the masses before they get reached by the virally spreading invitations. I look at this like in war, where you first fire with the artillery (PR) and then send in the infantry (invitations). Also in many cases there may be economic externalities, and reaching a critical mass will also have an impact on the virality coefficient. Therefore, the actual post-launch viral coefficient will be above one, which is all that matters.
Even though the ecosystem of web apps is unlikely a “zero sum game”, someone’s gain in virality is often away from someone else’s. This happens when everyone is optimizing ther virality in every possible way. Apps are fighting for eyeballs everywhere in feeds, emails, etc. The more there are players using the right methods, the harder it will be to reach a coefficient above 1. Therefore, one could assume that some apps would have to do a big PR launch just to get over the edge. I would be glad to hear what others think and what the new startups that recently launched a successful viral app did.