Yes, that's usually how it works with Netflix, Spotify and similar single-purpose services. But social media? If you take social network as a single-purpose service then there is no need for several or dozens of different similar services, because it wouldn't make sense as a social network, if you cannot interconnect them.

Suppose you pay a subscription for one social network, you'll need to pay also for the other one, to get connected to people on that other social network! It needs to be one big single social network and everyone pays for one subscription. Anything else doesn't make sense, but a single social platform as a network is unthinkable. So, basically, subscription doesn't make sense for social networking.

I don't understand your point here. Just like people pay separate subscription fees for Netflix and Hulu, which essentially provide the same service with different content, why wouldn't people pay separate subscription fees for Twitter and Facebook, if it were an option to avoid ads?

Because, in the end it would cost a fortune? Imagine that, most people only pay for one content provider.

A social network is not a content provider, just a different kind of social network. If you cannot afford a subscription of some social network, you miss out. And that's not acceptable in terms of social life of a human being. Rich folks would be able to roam all networks, poor people wouldn't. So, if you only can afford some "cheap" networks, the whole thing ceases to make sense, as soon as you are barred from accessing other networks. Exclusiveness in social media is an oxymoron. See where this is going?