I've been having the same problem, but I came to this thread because I noticed FB notification volume as the 2nd app to have this problem. My 1st app issue was with UberDriver when I noticed my in-app alert volume was extremely low. In-app settings have the alert volume maxed. When the alerts sound, adjusting the volume via the rocker shows it as "alarm volume" which is maxed out. When I first log in, I get a notification that I am now "online" and that notification is loud. The navigation voice is also loud. It's just the other in-app sounds that are barely audible. Google found me others with the same issue, but no answers. After I noticed FB notifications were also very low, I searched again and found this thread. I'm guessing some apps are having sound compatibility issues with Pie. I think the problem is that it's only playing the sound via the ear speaker and not the speakerphone speaker? Either way - it's not just FB experiencing this. Either the app developers need to figure out how to make it work with Pie, or the Pie developers need an update to make apps work as they did with Oreo. And yes - I checked notification settings in each app, have installed and reinstalled, and have updated both several times since the Pie upgrade.