ISPs are common carriers, requiring them not to do this. (Same thing with airlines, as another example; they would not be allowed to deny someone transport because they became aware that that person was participating in the Charlottesville thing, like Airbnb did.)

Google and Facebook are not common carriers, so they are under no obligation to be content-neutral. (If you're arguing that they should be common carriers, that's a perfectly fruitful discussion to have.)