Here's the difference: TV network made a deal with the government (FCC): they got a monopoly access to a limited resource (spectrum). In exchange they agreed

Here's the difference: TV network made a deal with the government (FCC): they got a monopoly access to a limited resource (spectrum). In exchange they agreed to limitations set by FCC, like decency standards. Limitations that go beyond what the law, as created by congress, requires. YouTube or Facebook didn't make a similar deal with FCC because they use unconstrained resource (internet bandwidth). Furthermore congress did the right thing and created a law explicitly calling out this scenario and made internet companies not liable for user generated content. Why did they do it? Because without that there would be madness.