Social networking sites question. Care to explain? (details inside).

They do not get a message, but if they used to be able to find you and now when they search they can’t, then they will deduce you blocked them. They might ask a friend you have in common if they can still see you, and when that person can, but the person you blocked can’t, they will feel confirmed that you indeed blocked them.

Also, you can tell if someone has limited your ability to see parts of their profile, because they’re kind of glaringly absent. So the absence of an “I just blocked you” message really only helps if the blocked person never looks at your profile in the first place… that is, not much.

On Facebook they will just get a message that says “XXXXX only shares information with friends. If you knowXXX…....................”

No, but it can be obvious in other ways. For example, if the person has suspicions, a mutual friend might confirm his or her suspicions.