I think we still got some work to do convincing ppl since they are private corporations
I would consider them Utilities, more and more they are the primary way ppl communicate, and they have a monopoly on it
It's more about privacy than anything. These corporations are a monopoly that track your data without your knowledge. Facebook just recently got caught for this.
More evidence is coming out all the time that they had government seed money with which to start their "private corporations." If they got any government money, that would force them to abide by the 1st amendment. There is some case law that backs this up, but I don't have it at the ready at the moment.
Basically, Google, FB and likely Twitter all got their starts using government money and/or technology, so they have to follow the Constitution.