i had notification on Facebook but it disappear when i logged in.why?

[Charles]

i had notification on Facebook but it disappear when i logged in.why?i never pressed on it to make it disappear

[R]

Ever notice how if you're on facebook and something happens, you get a notification near the bottom-left of your screen instead of the notification icon? If you log in right after something happening, sometimes it think the notification showed and so gets rid of the icon. If you click on notifications, it should still show up just fine.