Who Deleted Me is a free browser extension which can let you know when someone disappears from your Facebook friend list, and (maybe) why.
The program doesn't violate Facebook privacy rules or otherwise do anything sneaky. It just reads your friends list every time you visit, and lets you know what's changed - new friends and anyone missing - since last time.
Should you take this personally? Maybe not. Who Deleted Me checks to see if an account has been deactivated, and gives you an option to say that you unfriended this account yourself. But if neither of those are true, it looks like you've been ditched. Sorry.
Note also that version 0.4.3 of this extension was hacked to insert ads into your browsing. That's fixed now, but if you're using version 0.4.3 you should update immediately.