I would agree with you, but I've had 1 new friend join facebook in the last year or two. I've had about 5 leave. I also have a significant number of people who either never will join Facebook (due to attitude, principle or plain un-interestedness) and probably the majority who never log on, even if you send them a direct mail.

So I would be a pretty lousy friend if I just notified people within facebook and ignored my other friends.