So, I get what you are saying.. but what kind of monopoly tactics are twitter/facebook actually doing? In my view it's virtually nothing.
Let's ignore facebook since it wasn't exactly first to market with myspace before it (although it was first to market in "this has your real name attached"), and talk about twitter.
Twitter had a fairly unique idea, you can post public messages that are very limited in scope. They were first to market on that- and that's why everyone uses it. What is stopping twitter clones from popping up? Absolutely nothing. Many people who read HN could easily make a suitable twitter clone. But would anyone use it? Pretty much no. Why? Because people like to centralize their technology as much as they can to make it actually useful.
What a pain in the ass it would be if you had 4 different twitter equivalents that you were trying to use.. or try to remember "oh friend A is on twitter1, Friend B is on twitter3" etc.
Now, yeah people still do this do a degree of course.. some friends might be on facebook, others twitter. So of course this segregation still exists but it's generally for products which are different in scope.
Anyway- my point is it's people and the marketplace have decided "1 facebook is OK" "1 twitter is OK". Arbitrarily trying to break them up is not what people even want. Paranoid people might think that's what people want because all they can see is the possibility of censorship and security stuff, but 95% of people just want to use something that works and they can connect with people without everything being a pain in the ass.