The sound and popup notifications for Facebook calendar events are really excessive and annoying. I know you can turn off the FB calendars altogether, but is there any way to turn off only the notifications, but keep the calendar events showing when you navigate to your calendar app?
Right now, there doesn't seem to be a way to separate the two. In Windows Phone 8.1, the Facebook app acts as an intermediary between your phone and your Facebook account. So your events on Facebook are synced with the Calendar app on your phone and the Calendar app notifies you when it's time.
As you've already noted, you can turn off the calendars all together but you can't turn off the notifications. And sadly, that's true.
Have you tried this?
Go to Settings -> System -> Notifications + actions
Scroll down to Facebook and tap on it
Uncheck "Show notification banners"
Change "Notification sound" to "None"