We all use WhatsApp regularly, right? Apparently 450M people a month do, and that gave Facebook a few billion reasons to be interested. Founded in 2009, WhatsApp is a cross-platform, mobile instant messaging app, used to send text, video, image, and audio files. According to both parties, Facebook has agreed to buy WhatsApp for $4B in cash, $12B in FB stock, and another $3B of stock vesting over 4-years. More here.