Susan, I don't believe Facebook ever notifies you when someone has unfriended you.
E.g., I happen to know that a couple of my sister's friends who at one time were my FB "friends" both unfriended me because of our political differences, and the only way I know that is that my sister told me. (You can track the additions to & subtractions from your "friends' list if you want to bother, but I didn't.)

(As an aside, the irony about those two guys is that they both still "argue" with me on my own FB page from time to time! I allow "Friends Of Friends" to post on my page.)