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.