Facebook is currently testing a feature that would combine their family of apps, Facebook, Messenger and Instagram, together to show notifications from the other apps letting you instantly switch between them. This feature will help organize your notifications across all three apps especially for those with OCD about seeing the notification number on the corner of the app icon.
The feature was first found out by Mari Smith, a social media analyst and Facebook confirmed this by stating that that are conducting a small test to help connect with people and exploring ways to make it easier for people to switch between accounts. The app switcher will appear at the top parts of all three account which sums up the pending notification from all the accounts, and tapping on it will bring up a menu for you to select the apps that have those notifications.
As of now, there is no mention that WhatsApp, who Facebook also owns, will be included into this new feature, as this feature is still under testing, be expected to see it soon when it rolls out to others in the future.