Ok, I sideloaded the exact same book onto my Kindle and tried sharing and it worked fine. I tried again using the emailed version, same error as last night. So it's definitely the emailing of the file that breaks the sharing function. I assume it's Amazon adding the Whispersync functionality that breaks it.
For the failed attempt, Amazon email me saying "We were unable to send your message because of the following issue:
Something went wrong, please try again later."
Interestingly, the Facebook message showing after the share from the sideloaded book is is "Share from personal document" as opposed to "Share from <book name> or <author name>"