Popular cross-platform texting app WhatsApp reduced its iPhone app from $0.99 to free on Thursday. The app now delivers 10 billion messages per day, which is no surprise considering how consumers are choosing to use app-based messengers instead of paying for texting. WhatsApp is so popular, Facebook (FB) is reportedly interested in acquiring it. While it’s true the Android version of WhatsApp is also available for free, that version requires a yearly $0.99 fee after the initial year. Still, WhatsApp’s advantage over Apple’s (AAPL) built-in iMessage is that it works across multiple mobile platforms. At zero dollars, WhatsApp is a no-brainer download if you don’t already have it on iPhone.