I'd be puzzled....and a bit surprised. I mean it says something that after all this time they're still thinking of you! Maybe this person never found anyone else like you and is now regretting the end of the relationship. Maybe this person is regretting not fighting harder to keep you. If its definitely over with no hope of reconcilliation, then I'd probably ignore it and move on. I think that any sort of response would give them hope, even if you told them you could only be friends.