[robbles]

Great analysis - I didn't realize a small viral coefficient could have this much effect!

I'm really curious about the viral coefficients of other SaaS apps - I've seen a couple numbers thrown around in various blog posts, but never a comprehensive list. Has anyone compiled something like this?

[logvol]

Sounds like a great idea for a follow-up blog article. In fact, I just saw a presentation that said that before the referral program, Dropbox only had a K = 0.1. I'll try to put a list together.

Most of the models that we've seen are rather simple -- they assume that you just "have 1000 users" and then just show what happens from viral growth.

While those are illustrative of first principles, they don't really hold up when it comes to modelling your userbase over time.