What is the best, most polite, mature way to leave a social network?

Is it best to simply disappear?

Should the account be removed or just put on ice?

Are PM goodbye’s preferred over public goodbye’s?

If announced, is the announcement enough, or should reasons be given (lifestyle change, no time, new hobby)?

How long would it take for you to notice a friend missing from your favorite social network? What would you expect of them if they decided to leave?