Following its acquisition of Instagram earlier this year, Facebook is said to be eyeing up another popular service. Cross-platform messenger WhatsApp could be the next item on the social network’s shopping list as the company looks to extend its mobile presence, according to sources close to the matter.

Facebook CEO Mark Zuckerberg recently told Bloomberg Businessweek that mobile will be crucial to Facebook reaching the “next billion.” And what better way to reach mobile users than by acquiring the most popular mobile messenger.

WhatsApp was serving a billion messages a day by October 2011 — that’s 41,666,667 messages an hour — and it currently sits in the second spot of the iOS App Store’s paid charts, selling for $0.99. In fact, it’s almost always somewhere near the top of the paid charts.

And it’s not just available on Android and iOS; you can also gets WhatsApp on Windows Phone, BlackBerry, and Symbian devices. So no matter what kind of smartphone you own, you can use WhatsApp to chat to your friends for free. It’s no wonder the service is so popular.

It seems Facebook has noticed this, and according to a TechCrunch report, the social network is now in talks to acquire WhatsApp. “We’re still digging around on potential price and other details about how advanced the deal is,” the report says, but “sources close to the matter” have at least confirmed the negotiations.

Facebook already has its own Messenger, of course, but acquiring WhatsApp makes a lot of sense as the company looks to expand its mobile presence. However, Facebook’s ad-supported approach is entirely different to that taken by WhatsApp, which doesn’t pester its users with adverts at all.

How Facebook would tackle that if the deal does go through will be interesting.

• SourceTechCrunch