The total OPPOSITE of private messaging, usually reserved for those of us who cannot or will not use email, messages on social networking sites, or instant (and PRIVATE) messaging programs.
1) I wanted to speak with Kyrie so bad, but didn't know what to say in an email, so I public messaged her on MySpace.
2) I couldn't believe that jackass just public messaged me about the lame ass party he's having. I wish there was a way to block people from writing on my facebook wall...
3) If it wasn't for public messaging, I wouldn't be able to post those cute kittie pictures with the bad grammar anywhere!