How can I change the notification in my Android 2.3.5 device so that I can use a given wav file for sound notification when receiving a new Facebook message? If this is not possible with the Facebook app, is it possible with the Google+ app?
For using custom ringtones, I suggest Rings Extended app along with any file manager to browse the contents of your SD Card. This will allow you to pick any supported sound file to be used as a ringtone in any app.
In Google+ app, go to Menu -> Settings. Under "Ringtone" you can select the WAV file (pick "Rings Extended" when prompted, and navigate to wherever you saved the file), and "Notification settings" will give you finer control of which actions will trigger the notifications.
I don't use Facebook, but I imagine the app has similar controls in its settings.