I found somebody to go to a movie with because he posted on our film group saying:
"This film is playing in Tempe tonight, who wants to go?"
And I responded with a "Me".
The ads they show you will now be more targeted, about movies and located around Tempe. I've seen ads based on search terms in "private" conversations I've had on FB. They mine your interactions very aggressively. You'll almost certainly see that guy's posts on your "News" feed more often for a while.
"At any time"? Do you mean "With a court order"?
OK, I was a tad hyperbolic. But LEOs can search FB for "public" information about you and your friends without a court order. I put public in quotes because if you're not keeping on top of your privacy settings, you probably don't realize that some things you thought were private are actually public.