[tokenadult]

I'm just wondering why can anybody send messages on FB to others when they are not friends, and when their privacy settings is that only friends can message them.

Are you aware that they have the setting that only friends can message them because they told you so? Some people set their own profile settings to allow being messaged by nonfriends. For a while, at least, I had mine set to allow messaging by friends of friends. (Indeed, I see I still have my profile set that way.) It's up to each user to decide, and if I remember correctly, Facebook's defaults allow for more rather than less messaging. If indeed the person whom you messaged has a profile set to not allow messaging by nonfriends, and you (a nonfried) were able to message the person, that is a bit of a puzzler. Are you receiving replies to those message?