Check out this greesemonkey script!
First you need to use firefox for Facebook browsing
Second, you need to install this Firefox add-on
Lastly, install the script.
Then it'll grab a base line and tell you from there on out which of your friends disappeared from your friends list!
Check it out:
I found friends who deactivated their profile (because the script gives you a link to their profile) friends who have blocked me and rejected friend requests.
The one that blocked me, I viewed the profile from another Facebook account in another tab, switched back to my regular Facebook account while trying to send a friend request. Turns out Facebook knows better:
I even found a friend who deactivates her Facebook during the day, and turns it on my night. Funny!