Ask HN: Projections web app / FB app growth?

Hi HN,

I'm putting together projections for investors of a Facebook game we're developing.

Before I go and build it out myself, anyone know any good existing models/spreadsheets that take into account the viral rate, retention, lifespan of typical user etc?

I checked out the one at http://andrewchenblog.com/2008/03/05/facebook-viral-marketing-when-and-why-do-apps-jump-the-shark/ and it's good but somehow doesn't include how retention changes over time for active users, which I guess would be more like in the real deployment.

Any pointers appreciated, or case studies from existing FB apps.. Thx