[hlecuanda]

My hunch is that there is a critical mass in number of active users, that once achieved, Network effects make it more expensive to be left out of the network than opt in to it.

What the magnitude of such metric actually is, probably depends as much on cultural aspects of your target demographic as the quality and usefulness of the network provided services.

Surely there is an academic paper on the subject, but if there isn't, then the real question is Why?!