The article argues that all such apps are "swinging for the fences" and I don't completely agree with that. You can plateau a lot smaller than twitter and still benefit from delayed revenue, or alternative revenue models.
it's an obvious generalization... every app is going to be different depending on it's purpose and target customers/users. Your example of plentyoffish is perfect. A site/app like that DEPENDS on having tons of active users, in which case the fastest route to that is free.