That's somewhat true but if you think about it, every site really does have a single loop. Users that come, users that invite other users (any mechanism), users

That's somewhat true but if you think about it, every site really does have a single loop. Users that come, users that invite other users (any mechanism), users who respond. You can still think of it as an aggregated loop of all the flows you have that add to your viral distribution. Aggregate all functions for X, Y, and Z and that's your site's total viral factor.