Facebook is rumored to be interested in buying the WhatsApp instant messaging app for smartphones, as the social media giant looks to secure its future in mobile technology.

The website TechCrunch is reporting sources as saying the two are already in talks although the price and terms of the deal are unknown.

"Whatsapp, the multiplatform mobile messaging app that has been one of the runaway success stories for ad-free, paid services, has been in talks to be acquired by Facebook, according to sources close to the matter," said TechCrunch.

The site said emails for comment from Facebook and WhatsApp haven't been returned so there is little confirmation of the rumors. However, the purchase would make sense for Facebook which has increasingly looked to mobile consumers to grow its business.

The company's chief executive Mark Zuckerberg said in an interview with BusinessWeek in October that, after already reaching 1 billion active monthly users, it will look to devices such as smartphones and tablets for its "next billion".

"The big thing is obviously going to be mobile," Zuckerberg told BusinessWeek. "There are 5 billion people in the world who have phones."

WhatsApp has been a hit with smartphone users around the world because it offers a cost-free alternative to SMS messaging, using 3G and wireless network to send instant texts, including picture messages, with other users running the software.

It boasts users from over 750 mobile networks around the world running the iOS, Android, BlackBerry, Nokia S40, Symbian and Windows Phone platforms.

In a blog post in October 2011, the last time WhatsApp updated its numbers, the company claimed it had crossed its own milestone for the first time by "sending just over 1 billion messages in a single day".

"Just how much is 1 billion messages? That is 41,666,667 messages an hour, 694,444 messages a minute, and 11,574 messages a second," said the company.

If the rumors are true, it would follow a trend of Facebook buying small startups that have grown fast in the new era of smartphone communication.

In April, the social network bought Instagram for $1 billion - at the time its largest deal to date. Instagram allows users to take pictures with their smartphones, add filters, and instantly share them on social media such as Facebook and Twitter.