Ma Bell was really two monopolies -- long distance and last mile. Facebook is equivalent to the long distance monopoly, but that's the easy one. Federation and interoperability dissolves it.
The last mile is sticky because building a last mile network is very capital intensive and the cost is predominantly per mile rather than per user, so having two networks that each serve half as many users doubles the cost per user, which is very inefficient and creates a natural monopoly. There is no equivalent to that with Facebook.
...and end-user hardware.
Ma Bell was the Spanish Inquisition sketch of monopolies.