Belz...
Fiend God
I don't know a good term for it. Social Media is the catch-all term for these things but the reason I call them effective monopolies is that you can't just "create your own" and expect it to have anywhere near the same reach as those. This is before even considering possible issues of collusion.
Yeah but my point is that there are SO many alternatives if you want to talk with people online, whether it be friends, coworkers or complete strangers, that I can't call any of them a monopoly.