App To App
The biggest catch is that WhatsApp can only exchange messages with other people who have WhatsApp. Not a big deal, really, since Google Play reports between 100 million and 500 million installs for Android devices alone. And if that doesn't convince you, the chart at left shows the rise of WhatsApp message traffic since its introduction in 2011. One reason for its meteoric growth is its simplicity. Once a mobile number is registered, WhatsApp taps the phone's contact list and displays everyone on it who already has the app installed on their own phone. That removes all guesswork and simplifies setup to just about zero. For some people, though, that might also introduce a downside. Read on.