I disagree. I don't think virality has to be or perhaps even should be built in from the start. I think you should design a solution to a problem people have. And at that stage you should just choose the best solution, instead of ignoring solutions that don't have a viral aspect.

Furthermore, if you're sufficiently imaginative you certainly can take an idea and add a viral component after the fact. In fact, virality was bolted on to Hotmail, the application for which the term "viral marketing" was invented.

it so is a strategy.