David,
I'm really sorry for the delay in getting back to this. Between when you started this thread and your last post Facebook went an upended their whole API. The new
Graph API v2
changes how friend connections works quite a bit, and may make it impossible to do what you were looking for.. or at least, as powerfully as you'd want.
Until Graph API v2 was announced, when a user authenticated on your site, they had to grant permission to their full friends list (as well as other basic profile information) to your site/app. With that, you could make a call and fetch the user IDs of all of a user's friends and store them or do whatever you wanted.
With Graph API v2, the friend permission is an additional permission you need to request. That makes the authentication popup a little longer (which can decrease signups) *and* the user will be able to decline just that permission. Additionally, and most importantly, the friend list that you can retrieve for a user that's authenticated will *only* be friends that have also authenticated to the same application. Therefore, it would be a much more limited list than all of the user's friends, as available in Graph API v1.
Now, all new apps, created after May 1st *have* to use Graph API v2. If you created an app before May 1st, you can use Graph API v1 until May 1st next year.
With all that said, let me know if you're still interested in getting the user's friend list (whatever kind) and we can help. It's really just a few lines of easy code to get the list, but you'll have to do the dirty work after that to store, parse and compare to other user's on your site.
Thanks for your patience,
Alex