Apple (AAPL) just dropped a fascinating data point: 300 billion iMessages sent since the system debuted in the autumn of 2011. This is certainly an impressive number, but it also gives us a sense of how far ahead the tiny messaging startup called WhatsApp is right now. In August 2012, WhatsApp announced it is handling 10 billion messages a day. That equates to roughly 300 billion messages every month.

WhatsApp ramped up to that 10 billion-a-day pace in August 2012 from 1 billion a day in October 2011, which was right around when Apple’s iMessage system launched. Based on AppAnnie statistics, it is easy to see how the acceleration of the WhatsApp adoption suddenly picked up steam around the world in the first couple of months of the calendar 2012.

A year ago, WhatsApp was a top-5 app in more than 50 countries around the world. But by the summer of 2012, WhatsApp had become No.1 app in more than a hundred countries, beating all popular game apps. The only major markets where WhatsApp is struggling are China, South Korea and Japan, where domestic messaging systems were able to take root before the big WhatsApp adoption wave washed over the world in the first quarter of 2012.

What’s fascinating about the popularity surge of WhatsApp in iPhone app charts around the world is that it happened after the iMessage service debuted. The implication here is that iMessage wasn’t enough for iPhone owners. They needed WhatsApp to complement the service, possibly because so many of their friends and family were on rival platforms.

The most important messaging rivalry in the world is now waged between the biggest and most powerful tech company… and a tiny start-up with 30 employees.

Limiting Apple’s success is the proprietary nature of iMessage — it is limited to Apple devices. WhatsApp picked up formidable momentum just as Nokia’s (NOK) Symbian platform and Research In Motion’s (RIMM) BlackBerry range lost their grip, and its market penetration in smartphones is now topping 80% in countries like Netherlands.

The next 12 months of this race are going to be just fascinating to observe. Can WhatsApp somehow find a way to crack the three big Asian markets where it has failed to dominate? Can Apple drive adoption rates higher by launching substantially cheaper models in emerging markets?