Hold down the power button then press mute on the screen. Repeat to bring sound back again.

 

I had the same problem the other night. I use an app to silence everything but my ringer and alarm. My FB notification went off. I ended up switching to the "silent" ringtone for FB.

 

Android have multiple audio settings like alarm, notification, system, etc.
If you want to silence your fb just mute notification and set the loudness for ringtone.

You can access the settings easily by pressing the volume button up. An on-screen loudness control will appear. On it's right there's an icon for setting. Click it and set it up.

Hope this helps.

 

As long as notifications are turned down I don't get any audible alerts.

 

Sounds like FB is using ringtone instead of notification for their notification!

 

Yikes!

 

I'm pretty sure you're right with Facebook app using the ringtone setting instead of the notification volume. Any idea how to fix this? Even if it involves coding...