I'm having the same problem. A few notes in case this might jog someone into realizing what's going on.
I removed all the Facebook apps on my phone, disabled all of the facebook integration settings, rebooted it, and started from scratch ... multiple times. Even disabled the other calendar sources I have. (Google and work CalDav.) No luck.
If I go into Settings-->Facebook, turn on Contacts integration, and go back to the Calendar.app I can enable Facebook Birthdays and they come across fine. You would think that the iOS to Facebook authorization is handled in one spot but perhaps that isn't the case.
I've looked up and down the Facebook settings via the web site and the iPhone app. Nothing looks amiss. (See previous comment about authorization.)
I have, for lack of better words, messed around heavily with my iPhone Calendar and Contacts settings. I started with syncing with Google contacts and Googe Cal over the net directly, then moving to CalDAV and CardDAV, for both. At various times I messed with iCloud sync as well and adding the same Google services to my Mac laptop Calendar and Contacts app. Currently, I'm back to Google for CalDav and CardDav over the net on the iPhone and the Mac. Everything works pretty well except some Contact fields don't line up, which usually isn't a problem and, of course, Facebook events no longer show.
Why do I mention all that? Perhaps some of you have tried similar things and we've corrupted one of the internal settings files or plist and put our phones into an unrecoverable state. I can't recall when the Facebook events stopped showing after I messed with things because ...
The events stopped syncing but some of the old ones were around for a bit. They all dissapered once I started trying to fix it but it wasn't like they were all gone one day. When I noticed this problem it was because events I were adding stopped showing up but there were still a few listed. Almost as if the sync stopped kicking in but the events themselves were still cached.
Anyway ... that's probably tl;dr length but hopefully it will jog some brain cells.