Earlier quoted context omitted.
Here are a few use cases. For every one of them, I know at least one person (sometimes many) who falls into that use case. Guilds in online games often use FB groups to coordinate. If you want to spy on other guilds, you'll need a different FB account. I've known people with dozens. People create and abandon accounts. When they later decide that they want to use FB after all, it can be easier to create a new account…
Damn... the people in your life are very interesting, clever, and busy. I would have never guess most of those reasons. The people in your life sound like fun... and occasionally combative ;)
That said, I also think that people open up to me a surprising amount because I'm willing to talk and listen. You never know what stories you'll hear.
For example after 9/11 I had some interesting conversations with a woman who had been a minor princess there before the Russian invasion. She was the janitor who cleaned up the office I worked at at night. I believe that I was the only person in my company who was aware of her personal history.
Stop and think about that. I bet that you've seen a lot of people with interesting histories that you know nothing about.