Someone with whom I am not friends on facebook has messaged me through the site, basically telling me off over comments I posted publicly involving a mutual friend. They concluded in saying if I wrote them back, they'd consider it harassment. So my question is, given they took the initiative in contacting me, if I reply to them, responding to what they said and informing them that I am the one who doesn't wish to be contacted further, would I really be doing something legally considered harassment?