There are many many applications which utilize information about a person or a list of people — and most could probably benefit by making use of XFN. The Founders would be especially delighted to see one or more of the blog and blogroll creation tools out there adopt XFN and provide support for it. Perhaps organize blogs in a blogroll by friends, acquaintances, co-workers etc.
There are also many opportunities to build little components or bits of code that do interesting things with XFN. An advanced XFN builder could present a grid to edit a set of friends all at once, e.g. to set all co-workers to merely be colleagues, or select a whole bunch of people and indicate that you met them (perhaps all at the same event).