I want to delete all the events the were imported from my facebook plugin, then re-activate the facebook plugin, change the import frequency to every 24 hours and see if this fixes things for me.
You could try using a snippet such as this one to drop all Facebook-imported events. Please do heed the advice in the opening comment, particularly with regards to making a backup first of all and remember to remove it after you have finished using it or else all future Facebook event imports will be wiped out almost as soon as they are imported.