Except that the reason the US government wanted Shokin fired was because he was not investigating corruption and companies like Burisma aggressively enough. And it's not like Biden was even the one who came up with the messaging; he was just the one to deliver it. So advertisements stating or implying that the action might have been done to quash an investigation into Burisma (or more egregiously, into Hunter Biden specifically) are not exploring uncertainty in good faith, but are rather intentionally misleading.

Edit: Which isn't to say I think Facebook should be responsible for making these calls.