Facebook recently changed the referral tags for their event URLs, and our current URL normalization system doesn't recognize the new format.
Example: https://www.facebook.com/events/145813152724695/?ref_page_id=431240490226949&acontext=%7B%22ref%22%3A51%2C%22source%22%3A5%2C%22action_history%22%3A[%7B%22surface%22%3A%22page%22%2C%22mechanism%22%3A%22main_list%22%2C%22extra_data%22%3A%22%5C%22[]%5C%22%22%7D]%2C%22has_source%22%3Atrue%7D