I suppose that is true, but the reality is that most people have more Facebook friends than they have real friends, and one's real friends are less likely to actually unfriend them, even if that person is being annoying or offensive. Why? Because people don't want to hurt other people's feelings. Also, unfriending is really easy to do on Facebook. All it takes it a click. Unfriending in real life can mean anything from the easy (simply not returning phone calls) to the difficult (actually having a confrontation and saying you don't want to be friends).
But the real thing missing is that — and I can only defend this anecdotally — people do not tend to unfriend people who will actually notice that they have been unfriended. Example: I was promoting a book earlier this summer and several people told me they had hidden me from their news feeds because I was posting too much about the same thing, which I think is totally fair; I would have hidden me, too. But was I unfriended? Maybe, but if I was, I didn't notice, making the act a little bit meaningless in real world terms.
Have you ever unfriended someone on Facebook who you actually know and are friends with in real life? Have you ever noticed when someone has unfriended you?