Friend request
Facebook is very good at working out who should know who based on current friends and connections. Occasionally the links can be a bit tenuous. If you’re friends with Bob and his brother, then you might find Bob’s cousin from Atlanta popping up on your “suggested friends” list every other day.
Or you might find yourself with a rather random list of people you have mutual friends with. This girl decided to take a punt and send a friend request to one of these mutual friends, and he accepted, on a whim. Two whims and a few years later, and they’re married with a baby on the way. Facebook obviously knew what it was talking about in this instance.