Well it's not really FB per se. It's their stockholders. Their stock price has been plummeting as more and more issues arise and the fact that so many duplicate and fake accounts have slipped through the cracks isn't good for FB. It means that advertisers are reaching ~83million fewer people than they thought. It also shows stockholders that FB might not have it as together as everyone thought so they might dump their stock, which explains the plummet.
83 million is also only an estimate based on percentages of samples taken. It could be even more, although possibly less. A lower number seems less likely at this point.