No they don't. Those companies can't erase you from the internet, only from their own individual platforms that they own and operate. A "monopoly" would imply that without the use of these sites you could not be heard, but this is obviously false, not only because there are many other similar sites out there, but also because nobody is stopping you from creating your own blog or platform or whatever you like. No, your site won't be as popular as twitter, but so what? Your hyperlinks work just as well as twitter's.