As of like 10 minutes ago, now i have both Facebook and INstagram in my action center as notifcations. I didnt do anything, i know people have said that a notifcation has to trigger before getting added to the action center, there isnt anything you can do as a user in the settings to add.
So again, my suggestion is to post somethign and have people comment or like it and hopefully enough notifcations come through to finally trigger.
day 1, i had a couple of facebook notifcations and instagram, but didnt show up in notification center. So after two days, finally got a notification and it came through. So again, let it run its course and try to have your apps get spammed with notifications.