People like to look at the start of the sigmoid and extrapolate.

Before we started using "share" to mean "post" or "upload", and long before it meant "rent", it meant other things. Now it's so normal that we don't even think of how that word is a particular framing of those activities.

I’m quite confident this would have been based on metadata collected by Facebook from its user activity. The company has been extremely analytical about its own growth, and id be surprised if this wasn’t a conclusion from an in-house data scientist that just got Zuckerberg’s name stamped on it.

I looked up "psychohistory" and while the definition makes sense, I don't really understand what you mean by this comment.

While I’m sure Facebook had data to roughly justify this, just like Moores Law, I’m sure this is equal parts back-projection as prophetic declaration of intent.

For Facebook, they needed the cultural expectation to be ever-increasing data sharing. They (along with other companies) facilitated the creation of tools to share ever-increasing amounts of data.