As a private dick, I LOVE facebook. Most people will add damn near anyone who makes a friend request, especially if you tailor your dummy account to look like a member of their peers. From there, based on who they talk to, who they're friends with, etc etc, you use the info they thought was innocuous and harmless to try to predict where they'll hangout, what they'll do next, when they'll leave the house etc. Sure, I could find that info without FB, but it makes it a whole lot easier.
It's a job that gets easier every day.