I have added some social features to an iPhone game (Lexitect if you're curious), such as email, Twitter, and Facebook integration for sharing high scores. Along with these features, I am measuring how many times users make it to each step. The goal of these features are to make the game more viral, and I am trying to get to a measure of game virality.

I would think that a game virality metric would produce a number based on 1.0, where 1.0 = zero viral growth, and 1.01 would represent 1% viral growth over some unit of time.

How is virality normally measured, and in what units? How is time capped on the metric? i.e. if I gave each player a year to determine how many recommendations they make, I wouldn't get any real numbers for a year from the time I start tracking it. Are there any standards for tracking virality in a meaningful way?