Weird programming analogy that works for me: it went from being a library to being a framework. For your life.

As in, Facebook used to expect you to have to come and poll it. There was information there about the current state of things, and that information was replaced when people updated their status et al. You had to look, and look often, if you wanted to keep up. It was addictive, in both the good and bad senses of that word.

Today's Facebook is active, making your interaction with it passive; it keeps up to date on things for you, and notifies you when it thinks you would be interested in knowing something. You're the delegate module. You don't talk to it; it talks to you. You never actually have to check it or look at it.

Actually, for another analogy: today's Facebook is almost like a secretary. (It'd be one for real if it could guess your intentions well-enough to automatically accept/reject event invitations.) Like a secretary, there's no reason to go bother them. Nothing useful to be gained by polling. Zero addiction potential.

What's popular these days? Snapchat. Guess why?