interestingly, it seems that besides myself there are a goodly number of people wondering about the etiquette surrounding foaf friend declaration. while it's mostly a social and not technical problem, it's precisely the sort of thing that will keep foaf from reaching any kind of critical mass.
I've wondered a bit about this, too. If I've heard of you, can I list you as a friend? If I've emailed you once or twice? How about if I've dated your sister?
However, Eric Vitiello Jr. has an interesting schema for further specifying relationships in FOAF.shortname=ooodob