Consider this, if WhatsApp is worth 19 Billion to Facebook and depending on which reports you believe, 10B to Google, how is Bitcoin worth only 3 billion (rough market cap at 300/btc).

There are other instant messenger apps, but their mind/market share of 400 million users was worth a premium.

I personally think Facebook would have been better served by building their own app or buying a smaller startup with better tech and paying $500 to the 1% most interconnected users to switch (cost ~10% of what they paid). If that wasn't enough throw $10 to all of the rest of the users to switch and it's still half as much as they paid and they don't have a poorly developed, security hole ridden pile of legacy code to fix.