When the script detects that someone you used to be friends with is no longer one of your friends, a message will appear informing you about who it was and giving a link to their profile page.
The script checks by default once per hour when on Facebook, but this time can be increased through "Tools > Greasemonkey > User script commands... > Set time interval to check for removed friends...".
How Facebook Friends Checker works
* If the script has checked your friends in the past <<insert number here>> hours, do nothing.
* Otherwise, load a page on Facebook with a list of all of your friends.
* Go through each of these friends and add them to a list.
* Compare this list with the list saved the last time the script checked your friends.
* If someone was on the list before who isn't now, post the box informing the user.
* Save the new list of friends over the old one.
* Save the current time as the last checked time.
It is kind of like Quitter for Twitter.