Interesting idea.
Sites like FR would also be really easy to use to pass along messages utilizing steganography. Simply post a link to picture on a popular daily thread with a message embedded in the low-order bits. You wouldn’t even have to ping the person to the thread. If it were something like the Friday Silliness thread, or the current Undead Thread, your intended recipient could just browse the page anonymously and download the message. You could use the same picture to transmit multiple messages easily enough as well, just by updating the source GIF/JPG/PNG file. A refresh of the page would give you the updated file.