Rumor has it that Facebook is interested in buying the hugely popular cross-platform messaging service WhatsApp, in a move for the social network to “extend touchpoints with mobile consumers”. The ad-free messaging app has about 100 million daily users, with a presence in 150+ countries, sending some 10 billion messages a day as of last week (up from 1 billion per day in October 2011).

Those are certainly some solid numbers and even more so considering WhatsApp is a paid app. It just recently hit No.1 paid app spot in Apple’s US App Store, where it sells for $0.99, and has been topping the charts in several countries around the world for a while. WhatsApp is also a popular download on Android, BlackBerry, Windows Phone, and Symbian, where it’s free for the first year and $0.99 per year thereafter.

At this point there are no details on how much WhatsApp is likely to sell for, or when the deal might go through, if it goes through at all. Carriers have openly discussed the SMS volume erosion caused by WhatsApp, Facebook and iMessage. If Facebook does end up acquiring WhatsApp, the move would effectively combine two messaging systems that are already making some major advances in the mobile space.

WhatsApp is ahead of its competition in Europe, North America and Latin America. However, regional rivals like Weixin, Kakao Talk and Line are still powerful in key markets like China, Korea and Japan. These use a feature-heavy approach whereas WhatsApp has tried to keep things simple and focused on chat. What’s interesting is that Asian players have been very successful in monetizing their platforms beyond in-stream display ads, something Facebook might be interested in mimicking as it looks to diversify its business.