WhatsApp recently announced it surpassed 700 million users. Source: WhatsApp

WhatsApp is quickly closing in on 1 billion users, most recently announcing 700 million monthly active users less than 11 months after Facebook (NASDAQ:FB) agreed to acquire the messaging company for $19 billion. In that time, WhatsApp has added about 250 million net new users, and if growth continues at this pace, it could reach 1 billion users around this time next year.

One billion users is the threshold Mark Zuckerberg set when he told analysts on the company's third-quarter earnings call that "products don't really get that interesting to turn into businesses until they have about a billion people using them."

So, what could Facebook do with 1 billion WhatsApp users?

The current revenue model isn't worth $22 billion
When the deal closed for WhatsApp, Facebook had effectively paid $22 billion for the company due to an increase in Facebook's stock price.

When WhatsApp gets to 1 billion users, it will generate a maximum of $990 million from annual subscription fees, but likely significantly less as users don't pay the $0.99 fee for the first year, and some users (like me) were grandfathered in with a one-time purchase price of $0.99 on iOS.

Despite WhatsApp's huge user base, the company reported it lost about $230 million on revenue of just $15 million in the first half of last year. Churn is clearly an issue for WhatsApp, as many users are able to get by without ponying up the $0.99 annual fee.

Facebook must have larger designs for WhatsApp when it reaches the 1 billion-user mark. We should see the company push those developments in 2015 as WhatsApp nears 1 billion users. Here are a few things it could be working on to help monetize that gigantic audience.

Voice calling
It's no secret that WhatsApp has been working on voice calling. The company initially planned to release the feature last year, but that has been delayed with the expected release to come this quarter.

While I expect the voice-calling feature to be free for calls made between WhatsApp users, there's potential for the company to adopt a Skype-like service that allows WhatsApp users to make international calls to telephone lines for a relatively low price. Competing messaging app Kik already offers this service.

Considering WhatsApp is most popular in Europe and Asia, international calling could be a significant revenue driver for WhatsApp.

Enterprise services
The BYOD, or bring-your-own-device, trend is becoming more prevalent in the enterprise space. Instead of issuing employees smartphones for work, employers can subsidize employees' regular cell phone bills.

With more enterprises adopting BYOD, the market for cross-platform secure messaging and calling services is increasing. This is a market where BlackBerry has done well with eBBM, but the door is still open for competitors to take a piece of the market.

Facebook is typically looking to gather as much data as possible on its users, so the enterprises may hesitate to jump into bed with a company known for taking whatever data comes its way. But in November, WhatsApp rolled out secure end-to-end encryption for Android users. If the company can build on top of that momentum to flesh out a full enterprise suite, it could generate a monthly subscription revenue stream.

Payments may be the most straightforward way to monetize WhatsApp's growing user base. Last year, Facebook hired former PayPal president David Marcus to run its messaging products. His expertise in payments practically makes the feature a shoe-in to find its way into WhatsApp, Facebook Messenger, or both at some point in the future.

It wouldn't be the first messaging app to offer the ability to send money to friends. Snapchat rolled out the feature last year, and WeChat has been helping people in China send electronic funds to one another since 2013.

But WhatsApp, on its way to 1 billion users, has a network that surpasses all of its competitors. If you want to send money to someone, both parties must have the app, and WhatsApp is increasingly likely to be installed on both people's phones.

Things are getting interesting
Although Facebook just closed the acquisition of WhatsApp last quarter, 2015 will be a big year for the company to invest in monetizing the business. To its credit, Facebook warned analysts that its operating expenses will climb significantly next year. While the company had been steadily adding about $1 billion to OpEx during the last couple of years, next year it plans to increase OpEx by about $2.5 billion.

While not all of that additional spending will go toward WhatsApp, a significant portion will. And if Facebook can't get the business producing a profit by the end of the year as WhatsApp closes in on 1 billion users, profit margins at the company will suffer.