Clay Shirky hammers FOAF. For good reason. While attempting to precisely describe relationships may be good as part of a personal contact manager or enterprise analytical systems (which analyze server files and email streams and are expected to be inaccurate), they are entirely inappropriate for public distribution.
We recently had a bit of an HR fiasco. People didn't realize the difference between seeing something on the screen for 10 seconds and giving the ability to make authoritative copies with potentially unlimited distribution.
Personal relationships are the same way. It is appropriate for me to record in Outlook that someone is a "work acquaintance" and have Outlook make decisions based on that information (always use work email address, allow my work contact information to be verified). It is inappropriate to have that as part of a public FOAF file.
What I'm surprised about is the lack of FOAF hacking that has occurred. Where are the hacky screen scraping translators between social networking software? The great part about FOAF is that you can extend at will -- so if you really want UsedToBeFriendsWith, implement an application that demands it and provides clear user value. That's the beginning and the end of the argument.