Youtube, facebook, twitter should all be public utilities. The argument that they're private companies just doesn't hold water anymore when they have contracts with the CIA, actively spying on people.
Jimmy Dore explains it far better than I can.
When you say something controversial on your cellphone to somebody Verizon doesn't ban you from using their phones, that's how social media should be. They should be a carrier, not a arbiter of what can be said. Who watches the watchers?
So yes, when they block somebody that is a 1st Amendment violation. And possibly breaking anti trust laws as well.