-They enjoy a de facto monopoly and are protected by the extreme cost, risk and time involved in building competing services.
-Finally, they have a potential for abuse (say, with selective censorship or politically biased algorithms) that could essentially curb the Constitutional rights of individuals
If these points sound familiar it's because they're frequently used when arguing for the nationalization of a private company. Since I think that's (currently) out of reach, I believe regulating Facebook, Google, et al as public utilities to be the next best thing.