But Facebook, with >300m users, is an edge case. It's a business model in the same way getting into acting in order to become as rich as Tom Cruise is a busines

But Facebook, with >300m users, is an edge case. It's a business model in the same way getting into acting in order to become as rich as Tom Cruise is a business model. 37Signals model of charging for a service, on the other hand, can at least appear reasonably replicable (though also much harder than it seems, if you don't have Fried's natural talent for marketing & PR)