What if we were to identify your evangelists based on "cluster completion?" What I mean by this is that we'd look for clusters that go from being densely unregistered friend clusters to densely registered over time, and see which friend(s) were the catalyst(s).
From a purely consumer standpoint, I would use something like this to create my FB lists, if you made that easy. It's also useful when planning events. So, while I would personally never use it to send an app invite, I would use this if I was planning a party, and wanted to quickly see who I should invite and message, etc
We actually thought about offering this as a tool to a company like Paperless Post to speed up their process.
What do you think?