So you would be ok with a phone company that doesn't allow certains groups of people to make phone calls? For instance, people whose political views it doesn't agree with? Or even worse, people with certain ethnical backgrounds?

Of course now you might say, if such a phone company existed, those people who were denied service could just choose another one. But if this company would be the one dominating the market, this would mean they couldn't reach all the people they would want to anymore. This is the analogy with Facebook.