And how much would those coins be worth now? How much did it cost to make such a mistake?
Not sure how the acquisition of Whatsapp could ever be considered a mistake. Facebook not only own the most popular social medias around today, e.g Facebook, and Instagram, but they also own the most popular messaging application around. Remember, Facebook doesn't just make money by visitors they make money by selling data. Now, despite Whatsapp being encrypted on the user end, I almost guarantee that they collect certain data to better serve advertisements to their users. I don't even want to estimate how much Facebook are earning per day because of advertisements.
So, despite the flawed logic that was pointed above about not being able to buy more than is available in the market, its the fact that Whatsapp was a proven product, and wasn't that much of a risk. Bitcoin, is risky. It might be 50k today, but it can easily slide down that margin, just because of its volatile nature. Obviously, we probably have high hopes for Bitcoin, and sit on the more optimistic side of things, but Facebook is a business, and the risks they take are usually going to be calculated risks, instead of outright risks. Whatsapp was always going to make them money, Bitcoin wasn't guaranteeing anything at the time.
So, although it might have been a good investment for them at the time, it is by no means a mistake.