More than a few eyebrows were raised when Facebook (NASDAQ:FB) announced it was acquiring the fast-growing WhatsApp messaging app for a whopping $19 billion in February of last year. At the time, WhatsApp boasted "more than 450 million" monthly average users (MAUs) and was growing quickly: to the tune of more than one million each month. Even at that relatively early stage, Facebook CEO Mark Zuckerberg noted that WhatsApp was "on a path to connect one billion people."

Writing a check that size will always garner attention, but it wasn't just the amount of the WhatsApp acquisition that caused some head-scratching -- there was also the little matter of Messenger. Certainly, there wasn't room for both to grow to the extent Zuckerberg expected, right? Turns out, Zuckerberg's predictions for WhatsApp and Messenger were spot on, and according to a recent study, the fast-growing apps may even surpass Facebook itself in sheer size before long.

Survey says
As it stands today, the top five messaging apps combined have over 3 billion MAUs, and the majority of those count Facebook's two properties as their app of choice. WhatsApp alone is nearing the vaunted one billion MAU plateau, doubling its monthly user base since Facebook acquired it, now sitting at over 900 million.

Facebook's Messenger is hardly a slouch, having more than doubled its own MAU count in the last year to over 700 million. Positioning Messenger as a stand-alone app, much like WhatsApp, has gone a long way toward its meteoric jump in users. Not surprisingly, given the advent of smartphones across the globe, many of WhatsApp's and Messenger's MAUs are of the mobile variety. To put that into perspective, excluding its SMS Fast Follower pseudo-users, Twitter (NYSE:TWTR) has just 307 million MAUs -- about 80% of which are mobile -- equal to a paltry 8% increase from a year ago.

In other words, WhatsApp and Messenger have doubled in size the past year or so, while Twitter's MAU growth has been stagnant at best. Clearly, Facebook's prized properties are filling a niche that Twitter simply can't fill, and it appears the disparity will continue to widen.

As per a recent report from research company eMarketer, nearly half of today's three billion global messaging fans -- an estimated 1.4 billion to be precise -- will use a mobile phone to connect with friends and family in 2015, a more than 30% jump from last year. Those 1.4 billion monthly mobile messagers are an estimated 75% of the world's mobile phone owners -- and the trend is expected to continue.

In just three short years, eMarketer suggests that 2 billion messaging app users across the globe -- about 80% of the world's mobile phone owners -- will use their devices to send and receive messages each month. Based on eMarketer's predictions, at their current pace, both WhatsApp and Messenger could even surpass Facebook itself in sheer volume.

What's the big deal?
Those are big numbers, but what do they mean for investors? Zuckerberg has been asked numerous times what the monetization plans are for WhatsApp, and his response has been consistent for the past year: Facebook will explore revenue alternatives once it reaches the 1 billion MAU level. At its current pace, that will likely be by year-end.

As for Messenger, plans are to transition away from a messaging app and transform it into a "gateway to the mobile web." Facebook has been a bit more elusive in answering Messenger monetization questions, but as it approaches the 1 billion MAU plateau, you can bet the question of how Facebook intends to turn Messenger's users into revenue will move to the forefront.

Last quarter, Facebook said it had 1.55 billion MAUs, a 14% increase from last year, and nearly double the rate of Twitter's user growth. As impressive as Facebook's growth is, with the fast pace of adoption WhatsApp and Messenger are enjoying, along with eMarketer's mobile expectations for the coming years, Facebook may soon be the third most popular app in its arsenal. And when monetization follows, Facebook shareholders will reap the rewards.

Tim Brugger has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Facebook and Twitter. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.