When AT&T was a monopoly and you had to rent your phone from them at an outrageous price, you could also walk away. But having to either walk/drive around or send slow mail to communicate was not very efficient. Perfectly doable, mind you, but very inconvenient. Sometimes the cost of walking away is just too steep, so abusing a dominant position becomes very easy and tempting.

I don't think Twitter or Facebook are anywhere close to that threshold. Maybe that's just because I barely use them. But I'm sure some people were also not using phone in the seventies, so I won't conclude based on my individual case.