WhatsApp has seen impressive growth over its few short years of life. 450 million users is impressive, especially given that Facebook itself only has twice that number. Can WhatsApp grow more? I've read plenty of analysis that suggests that the best way for WhatsApp to grow next is to add additional services around its core product, rather than go after more users. This seems to suggest that anyone who is potentially a WhatsApp user is already a WhatsApp user, or a user of an alternative service such as WeChat.
WhatsApp in its current form competes with SMS, and wins because it is both cheaper and better. But it relies on a phone that has both apps and data. Most of the world does not have this type of phone, yet. But significantly cheaper communication might be enough to warrant the purchase of a smartphone, even if that phone cost many times a monthly salary. The same was true with basic phones just a few years ago. If WhatsApp plays its cards right it could be the main reason why someone buys a smartphone. That market is easily 3 billion people who all already have feature phones. And this is why Facebook cares. Because WhatsApp is the killer app for smartphone adoption, and smartphone adoption is good for Facebook.