With a humorous bite Becca Hafter writes at The Awl:
But Olds don’t have Facebooks. They’re “on” Facebook. The Olds need a preposition when they talk about Facebook because they do not automatically locate themselves on the Internet. But their word choice serves an additional function. Their constant verbal reification of operating in (on) a space that is not theirs but Facebook’s works pretty well to keep themselves aware of the fact that Facebook owns everything they post. This is something millennials love to disregard.