Debt and pension liability
Instagram, being a private company have little or no debt or pension liability. So the billion that Facebook buys what they think is 1 billion of worth.
Washington Post Media is loaded with long term debt and especially large amount of pension liability. Sure they have lots of real estate properties that Bezo could potentially leverage to pay off, but still, the $250 million doesn't exactly reveal the true cost.
WPM manged to suckered Barry Dillard into buying Newsweek for $1, and Dillard realized much too late that he overpaid due to debt and pension liability. Dillard have sold it to some other sucker.