I agree with the general sentiment that traction beats a prediction, however, I think its valuable to understand the relationship between the key metrics before you start.
Thats not to say that your model will match reality, but its certainly valuable to understand the dynamics involved.
I'm working on a series of blog posts that develop a virtual world business model. However, the principles also translate to freemium & social business too.
I recently wrote a post (with google spreadsheet) illustrating how the addressable market size (network carrying capacity) effects the viral growth of your application.
The theory goes that as the network becomes saturated the viral invites, paid acquisition, etc starts to fall on players who have already evaluated your service. Over time the success of your marketing and viral actives decline.
Check out the post at: http://dubitplatform.com/blog/2009/7/30/calculate-how-the-ad...
The google spreadsheet linked in the post illustrates how I've set about modeling viral growth, paid acquisition, ARPU, churn, and so on.
The model applies to any viral application - but its worth noting that products relying on advertisements can expect a significantly lower ARPU, knocking into your paid user acquisition.
You may also find this post interesting: http://dubitplatform.com/blog/2009/7/26/calculate-how-much-y...
It goes through the relationship between ARPU, LTV, and how your viral coefficient effects how much you can afford to spend acquiring a new player.
I think paid acquisition is a under utilized strategy within social networks - Zynga aggressively bought FBK users.
Hope it helps!