How Facebook CEO Mark Zuckerberg courted WhatsApp founder Jan Koum

The relationship between Facebook and WhatsApp started in the spring 2012 over coffee at a German bakery. It was consummated on Valentine's Day with chocolate-covered strawberries, after just five days of talks.

Facebook, the world's largest online social network, agreed to acquire mobile-messaging start-up WhatsApp for as much as US$19 billion in cash and stock, as it seeks to expand its reach among users of mobile devices.

The deal is by far Facebook's largest, bigger than any that Google, Microsoft or Apple has ever inked. And its boldness demonstrates that Facebook wants youth, a wider footprint in more markets and new kinds of services to stay fresh.

Analysts say the staggering price tag highlights Facebook's drive to be more mobile, global and to remain innovative.

"It shows the continued determination of Facebook to be the 'next' Facebook," says Benedict Evans, a partner and analyst at California-based venture capital firm Andreessen Horowitz.

But the deal was likely to raise worries that Facebook and other technology companies were starting to become overzealous in their pursuit of promising new products and services, said Anthony Michael Sabino, a law professor at St John's University in New York.