100% incorrect. Facebook Notifications are fast and consistent on Android, iOS, and Blackberry. It's not a Facebook issue, and I'm pretty sure the push notifications on WP7 do not come directly from FB. Notification speed and consistency on WP7 is generally slow and volatile. It's already been demonstrated several times by myself and others.
@ OP Redo your facebook connection. Last time I did mine Windows Live asked for an additional permission regarding facebook notifications. It won't make them faster, or more consistent, though, from what I've experienced. I think it gives the phone the ability to clear notifications you check. But at this point, it's worth trying anything.
It probably hasn't stopped, it probably is just slow. Facebook for Windows Phone is usually 5-10 minutes behind other platforms in getting the PUSH Notifications to the user. I wonder if Microsoft's PUSH servers are overloaded? This has been an issue pretty much since launch.