Is the public post written by the questioner with the intention of making the young man understand that she will not contact him considered a violation of the oath she took—under duress—not to contact him by any means?

The relationship between a woman and a non-mahram man is forbidden. Remorse is a condition for repentance, and all means that could lead to this forbidden relationship must be blocked. If the oath included not communicating with the young man, then communicating with him necessitates breaking the oath, unless sending the message was not intended by the oath. Repentance is also required for the words mentioned in the oath.