Serfs got protection (and other services, like justice! uh.) from the feudal lord. I don't like Varoufakis' typical oversimplification, but this one doesn't seem that bad.
Of course other important aspect is that even if you are free to move to a different platform - in contrast to serfs, who in general had to ask for permission to move - you can't move your followers. So in that sense every second of effort put into a twitter profile is improvement to the land that you can't take with you. And, as we know, while brands are important users are lazy and stick to platforms much more than to brands. (While, again of course, there's usually a very devoted core community around content creators, but that's not true at all of most brands, and especially not true of regular users who just want to have some Reddit or forum-discussion-like experience.)