I'm wondering the same thing! I used to be able to see ANY notifications, no matter how long ago I received them! Now I only have them from the past week! 7 days to be exact! Why in the hell would I have to download an app to be able to do what used to be a standard feature on Facebook! What if I haven't checked my notifications for a month? I don't want to fish through all of my emails (sort though 100's of spam messages) to find 2-week-old notifications... When did this change? I swear it happened in the last month!
To view older Facebook notifications you should Search for ArchiveBook where you search for friends and it will come as an application. Join it and Facebook will show you how to retrieve older notifications.