I don't really use facebook much but when in the app if u hit the menu button there is probably an option for settings and in settings I'll bet there are notification settings which is what u would want to edit
I think u can do that from within the music app go into ur music app and long press a song u want there should be something like set as notifcation ringtone
Hmm well that's the only way I know to do it if ur nonrooted if ur rooted u can just move the music to ur notifications folder which I think is at /system/media u can just move ur own sounds there with root explorer and they should then appear after a reboot
You can make custom ringtones from your music collection on the SD card with an app called "ringdroid". Its a simple but nice app and user friendly. I actually have used it myself until I have been leaving my D2 on silent or vibrate lately haha