It sounds like someone on facebook has added your email as a contact for you without you or possibly them knowing about it. If you 'post' or send something on fb its added to something called a 'superwall' this occurs even if the sender deletes it - its still recorded forever. Since you have no fb i am presuming that it is sending you the messages direct to your email instead.
I attend meetings as an Artist at NSA studios and we have professional and personal fb as one way of giving us exposure. There are some people that don't 'do' fb but there email address is recorded so they get the same info etc and are not left out.
Sorry for the long winded explanation, get your mates to check the settings on fb.
Peace
