Actually? I sort of think that facebook's impressiveness is in its ability to scale the features so nicely.
I don't think there are any features on facebook that couldn't be made by your average ycombinator news reader (they might be slow as all hell, but I bet most of us could create working demos of most features).
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).
