"We are sorry about the downtime," wrote Koum. "It has been our longest and biggest outage in years. It was caused by a network router fault which cascaded into our servers," Koum added.

"We worked with our service provider on resolving the issue and making sure it will not happen again," Koum he further wrote.

WhatsApp was down for more than three hours on Saturday just days after Facebook bought it for USD 19 billion. Facebook is betting huge on mobile with the eye-popping cash-and-stock deal for WhatsApp, which was only started five years ago but has quickly grown as a free alternative to text messages.
It is Facebook's biggest acquisition and comes less than two years after the California-based Internet star raised USD 16 billion in the richest tech sector public stock offering.

The five-year old company currently has about 450 million users worldwide and is the leading smartphone-based messaging app.