One thing to note is that on a large/complicated site, you can have 10's or 100's of possible engagement loops(what most people call this) which this metric applies to. The article tends to talk about optimizing a single loop. The most common engagement loop, I've seen, is making users confirm their email address.
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.
