This rather common etiquette breach is puzzling. Why would anyone want a private message splashed all over a public place like Facebook? For some, it's a lack of understanding in how the social media platform works. Remember that when you post something on someone's wall, potentially everyone can read it, depending on the person's privacy settings. So don't write, "Hey, Amanda still in the dark about her surprise party?" on the wall. Or, "Wanna golf at 2? I'll tell the wife I'm working late." And certainly not, "Let's get it on tonight, baby!"
Some people post private messages on the Facebook wall because they're too lazy to pick up the phone or use the Facebook message feature, which acts like email. And frankly, they don't care if someone reads their "private" note. Well guess what? We do! It's like being forced to listen to someone loudly yammering on their cell phone in an elevator. You don't want to hear their stupid conversation, but you're trapped. If you post private messages, we'll see them whether we want to or not. Or whether you want you want us to or not.