The premise is flawed. It would take more than popularity for Facebook or Amazon to become a monopoly but they would need to engage in illegal action protective of their market share. It's not illegal to be a monopoly; it's illegal to abuse one's monopoly position. When a competitor to Facebook, Amazon, etc is only a few mouse clicks away then seriously calling any digital property a monopoly is a stretch.
And this ignores that online monopolies don't seem to last very long anyway (Friendster, MySpace, Yahoo...).