In addition to features such as the "News Feed," individual user walls and the ability to upload photos and video to your personal profile, Facebook has a private messaging system, with some features similar to traditional email. If you need to send a file to someone using Facebook, attach it to your Facebook private message just as you would if you were sending an email.
Sign on to Facebook. Click the "Messages" icon, which is located on the left side of the site's top navigation bar and denoted by two "speech bubble" icons. Click "Send a New Message."
Enter the name of the person to whom you want to send the file, then a message explaining the contents of the file. Click the "Paperclip" icon in the lower, right corner of the message window.
Browse your hard drive for the MP3 file you want to send, then click "Open." Click the "Send" button to send your message and the file.