You are just a brain in a biochemical vat being fed electrical signals through the sensory organs.

Are they false signals? There are non-negligible priors that support his accusation, such as gag orders and a proven and recent history of lies and obfuscation. I personally think the simpler model of Facebook telling the truth holds more weight -- to the point of a conspiracy of this magnitude sounding more than a little silly -- but there will always be a shadow of doubt to some, and not fully without reason.