Try one of the ff:
1.) Try to disable facebook calendar in calendar app (calendar app>settings>calendars>untick facebook)
2.) From homescreen, going to menu>settings>accounts and sync>facebook. Remove calendar sync.
3.) Sync to facebook app, not facebook.com. From homescreen, going to menu>settings>accounts and sync, you will see your facebook account logged in. If you go to add account, you will see two options for facebook sync (if you have facebook app installed). One is to facebook.com, and the other to the facebook app. Use facebook app sync