Of course they should not. Or should they? We live in a free society and you are voluntarily participating. Or not. It's your choice. And that's wonderful.
People do not realise, how simple it is to stop. As with cigarettes the real trick is to just not do it anymore. You need some willpower and some plan to cope with the temporary withdrawal effects.
Single datapoint: I briefly used FB for a few weeks about ten years ago, but never saw its value. Today, thanks to dusty email and many other (more open) sites, I have a growing number of friends around the globe and locally.
There are places (like Central Asia), which are already operating in a Free Basics mode in that basically anything beyond facebook is an extreme hurdle to use. But anyone in the western world with lots of choices can stop feeding FB today, if they cared.
However, Facebook really is, at this point, a clear monopoly. The reason I use facebook is because that's the social network that everyone else uses. If I want to see baby pictures, hear about my friend's band's upcoming show, get updates on how people are doing after the norCal fires, or get trolled politically repeatedly by my insular political bubble, Facebook is the only place where I can do this. The next best network is nearly useless to me - not strictly because it's worse in terms of features or reliability, but because nobody else is using it.
I can't say it's absolutely impossible to do this without facebook - I agree that people can, with effort, patch it together, as you've done.
However, I think the "voluntary" participation aspect you've identified, while not entirely inaccurate, should recognize the coercion that results from a monopoly.
But here's a point: The zero-cost marginal cost characteristics of many software businesses makes this sector prone to be monopolistic. Acquire: data, users, or intellectual property and from there, competition will have little chance until a new wave washes clean the shores or you mismanage your company.
So the only way to stop this is to - do what? The political will and instinct, that would be required to redirect technological progress by not allowing monopolies is enormous. Not to mention the economical dynamics that come from companies competing on how to destroy privacy faster.
And therein is the fallacy. Facebook might be popular, but they are absolutely not the controller of all means of socializing. Everyone I'm friends with on Facebook, I also have the email (and often phone, and often other IM service) contact info.
You confuse generalized control with control in their own walled garden. Anyone is free to step outside of that garden whenever they please.
In any case, the next big thing is usually not just a better version of the last big thing.