Yours are set up so that only people you have added as a friend can leave/read your messages. Under the Settings view there is a "General Settings" link on the left. About halfway down it says "Visitor Messaging" in bold. There you can either turn them Off/On or set them to Contacts only.
“Everybody has a secret world inside of them. All of the people of the world, I mean everybody. No matter how dull and boring they are on the outside, inside
them they've all got unimaginable, magnificent, wonderful, stupid, amazing worlds. Not just one world. Hundreds of them. Thousands maybe.” -Neil Gaiman