Late yesterday, Facebook acquired WhatsApp for the princely (and ludicrous) sum of $19 billion in cash and stock. This is one of the largest acquisitions in Silicon Valley history -- and yet, if you're American, you probably don't even know what WhatsApp is, let alone why it's worth $19 billion. In a sentence, WhatsApp replaces text messaging (SMS) -- and it has grown to 450 million monthly users in under five years. While a purchase price of $19 billion might seem like insanity, especially when compared to its $1 billion acquisition of Instagram, it's actually a savvy move to ensure that Facebook remains the world's predominant messaging platform.