If Google instituted a $0.99 monthly fee to use Google search, people would line up to pay it. They don't have to, because their business model works differently, but they could do that, and it would work. I'd probably pay for an account.
If Facebook instituted a $0.99 monthly fee to use that, a lot of people would do it just because it keeps them in the loop with their friends. And in fact, many people would be willing to pay $2.99/mo for a business account which gets to do more (like post more events or whatever). Again, this isn't their business model, but this would totally work as a business model.
People would pay for Uber de-facto; they do pay for Uber.
Likewise, half my coworkers have bought coin packs in Pokémon Go. In general, lots of games do pay models.
If Twitter added a $0.99/year fee, there would be a clone written in a more scalable architecture taking all their users within a week. Some businesses would keep Twitter accounts, but that number would decrease as they lost users. Remember that Twitter added some minor advertising and instantly started losing users.
There are tons of other startups with this problem. The idea these days is, get a bunch of users and introduce ads. The problem there is that when you add ads, you're fundamentally changing your service. It's a bait-and-switch. And once you do the switch, if people weren't willing to pay for your service before, they're not going to want to start paying for it in the form of ads.