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There are many articles online that suggest this to "get over" an ex so you don't see them. I guess if it was painful for one they may want to "move on" quickly to get over you. I personally see blocking as hostile. Unless someone is very bad and say, abusive or a stalker. I think unfriending might be more appropriate especially if both parties agree to part ways and they just don't get along. If it's really "done" why be friends on Facebook. The "surprise blocking" by one seems very mean. Usually people will do this with no explanation or final good bye letter.
I think it means the other person is so upset that they can only cut you off like you don't exist because that is the only way they can "get over you".