Facebook has reached a definitive agreement to acquire WhatsApp, a growing cross-platform mobile messaging company, for a total of approximately $16 billion, including $4 billion in cash and approximately $12 billion worth of Facebook shares. The agreement also provides for an additional $3 billion in restricted stock units to be granted to WhatsApp's founders and employees that will vest over four years subsequent to closing.

WhatsApp has built a real-time mobile messaging service with over 450 million people using the service each month. Its messaging volume is approaching the entire global telecom SMS volume and it is currently adding more than 1 million new registered users per day.

"WhatsApp is on a path to connect 1 billion people. The services that reach that milestone are all incredibly valuable," said Mark Zuckerberg, Facebook founder and CEO. "I've known Jan for a long time and I'm excited to partner with him and his team to make the world more open and connected."

Jan Koum, WhatsApp co-founder and CEO, said, "WhatsApp's extremely high user engagement and rapid growth are driven by the simple, powerful and instantaneous messaging capabilities we provide. We're excited and honored to partner with Mark and Facebook as we continue to bring our product to more people around the world."

Facebook said that WhatsApp's brand will be maintained; its headquarters will remain in Mountain View, CA; Jan Koum will join Facebook?s Board of Directors; and WhatsApp's core messaging product and Facebook's existing Messenger app will continue to operate as standalone applications.

The deal provides Facebook entree to new users who eschew the mainstream social networks but prefer WhatsApp and rivals.

Google had also offered to buy WhatsApp for $10 billion, according to a Fortune report.