[goldins]

Interesting - I had the same thought not too long ago. I talked with a friend, and it turned out that a lot of the same friends that show up on my profile when I am looking at it show up for him when he views my profile.

As to (a) and (b), the friends that showed up seemed pretty random - I interact regularly with less than half of them. It's definitely not random, but I'm not sure how it's determined. It would be interesting to track it over time (as friendships are added, removed, and changed).