I think companies have seen what happened with Twitter and it has convinced them that they can try more drastic revenue generation strategies with little repercussion. They have all become strong monopolies in their respective domains and users who have grown up with the current offerings are not willing to put up with lesser alternatives.
The internet is basically ~10 websites for most people, only occasionally veering off the path to find some one off information. The casual user sees no reason to put up with the growing pains of alternatives and will put up with a lot from Google and friends if it means not having to create a new account on another website with no content.
How can you possibly replace YouTube and Reddit? Their value is in their user base and it's impossible to replicate that type of "success" overnight.