Facebook is apparently mighty impressed with WhatsApp, a messaging app for smartphones: Mark Zuckerberg's company announced today that it plans to buy WhatsApp for up to $19 billion in cash and stock. WhatsApp claims 450 million monthly users, and the deal is further proof that Facebook "is willing to pay huge sums to acquire rival apps that gain sway with consumers," says the Wall Street Journal. WhatsApp will continue to operate independently. Investors were apparently not so wowed—Facebook shares dropped about 5% in after-hours trading, notes MarketWatch. (More Facebook stories.)