The only people I've known to do such announcements are people I've met in games such as Peel a Meal, Restaurant City or stuff like that. Since the numbers can go up really, really fast. Once you no longer play a game or once the connection is no longer necessary, I have seen some people post an announcement, and others who wanted to stay friends and continue catching up would post on said person's status.
Other than that, if I defriend for personal reasons, I do not say anything. Sometimes I'll tell some mutual friends why so that they can use the information as they want, just in case the defriended person tries to bother them, but other than that, I defriend and that's it.