It's not a crime but it's for personal safety reasons for the poster amongst other things.
In this day and age unfortunately there are those out there that aren't particularly 'nice'.
Cyberstalking, etc. are well known and recently I know of one case where a 13 year old girl who posted her photo and became involved in private chat, met with the man, end result he's now up for aggravated sexual assault on four counts. That's just one example.
It's never a good idea to give out personal details, not even to those who private message you.
