Facebook does a lot of almost invisible but extremely smart things. Two examples: when you specify the type of relationship when adding a friend, Facebook uses

Facebook does a lot of almost invisible but extremely smart things. Two examples: when you specify the type of relationship when adding a friend, Facebook uses the info to decide how much information to present about the person in your news feed. Or when two people list themselves as being in a relationship, Facebook will show the most recent picture of the two together (if such a picture exists, and privacy permissions permit it in your case). Nothing fancy. Just extremely well thought out.