I don't think they would do well if they switched to a pay-only system, but they might offer special features as subscriber-only or put limits on certain things

I don't think they would do well if they switched to a pay-only system, but they might offer special features as subscriber-only or put limits on certain things in the free tier. If they offered no or limited ads and a simple chronological feed from friends in their paid tier, they might do well. Sort of the "get everyone in the world addicted to your crappy product and then make them pay to get a reasonable version of it" model.