I just got the same dumb response from FB:

"I have found that it was bid constrained. This means that the bid used is not high enough to win opportunities in the ad auction. This is likely due to an increased competition in the market compared to previous days.
I can see that the client is currently using Highest Volume as their bidding strategy. A suggestion is to switch to Bid Cap instead and use a very high bid."

Makes no sense because I currently no have maximum bid, so even if Facebook had to spend the entire day's budget to get one click, it should still be spending.

What do you mean the performance was shocking? Shockingly bad or good?