Facebook's Mark Zuckerberg.

Mark Zuckerberg once suggested Facebook Inc. (FB - Get Report) would start monetizing Facebook Messenger after the messaging app topped a billion users. With Messenger now above 1.2 billion monthly active users (MAUs), Zuck's company is gradually starting to make good on his prediction. And WhatsApp, which has amassed a similar number of MAUs, might not be too far behind on the monetization path.

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On Tuesday, July 11th, Messenger exec Stan Chudnovsky disclosed a beta test through which Messenger home screen ads have been shown to Australian and Thai users would be rolled out worldwide in the coming weeks to a small percentage of users. The ads have much in common with news feed ads shown on Facebook proper, and typically feature a photo, text and a link urging a viewer to do something like shop for products, learn more about an item, sign up for a service or start a Messenger conversation with a business.

Importantly, given how Facebook towers over the social advertising landscape, Chudnovsky says the ads leverage the same targeting tools available for Facebook and Instagram ads. And from the looks of things, it shouldn't be too hard to repurpose certain types of news feed ads for Messenger.

In addition to the home screen ads, Facebook has experimented with letting businesses that a user has already interacted with pay to re-engage them via "sponsored messages" that appear in Messenger. And though this is a more indirect means of monetizing, Facebook has long given news feed ad buyers the option to include a link in their ads that lets users message the business.

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Though Zuckerberg and others at Facebook once voiced skepticism about using ads to monetize messaging apps, Chudnovsky is singing a very different tune today. "[Advertising is] not necessarily everything, but it's definitely how we're going to be making money right now... there are some other business models we are exploring as well, but they're all around ads one way or another," he told VentureBeat.

One of those "other business models" could involve indirectly monetizing transactions. Facebook Messenger's bot platform began supporting payments last September, and it's easy to see Facebook giving creators of commerce-related bots the chance to promote their bots via ads. Likewise, developers supporting Facebook's Instant Games platform -- it works on both Messenger and the news feed -- could buy ads promoting titles that are monetized via in-game purchases (Facebook gets a 30% cut on them).

WhatsApp, which prior to its acquisition by Facebook published a blog post titled "Why we don't sell ads," remains ad-free for the moment. But some moves made over the last 12 months suggest it's only a matter of time before this changes.

Last August, WhatsApp updated its terms of use to let businesses communicate with users - cited examples included "order, transaction, and appointment information, delivery and shipping notifications, product and service updates, and [emphasis added] marketing." The company also revealed it would begin sharing phone numbers with Facebook proper, which Facebook could then use to provide friend suggestions and improve its news feed ad targeting.

Since then, WhatsApp has begun testing a platform for letting businesses message users, and has been surveying users on the degree to which they use its app to talk with businesses. And last month, the company was reported to be planning to migrate its services from IBM's cloud data centers to Facebook's, a move that could ease integration with Facebook's ad infrastructure.

Both Messenger and WhatsApp, meanwhile, should be able to monetize their recently-launched Snapchat Stories clones, Messenger Day and WhatsApp Status. Instagram is already selling video ads for its Stories clone, which now has over 250 million daily active users (DAUs). WhatsApp Status had 175 million DAUs as of early May, while Messenger Day (from the looks of things) has struggled to gain a following.

Facebook's new Camera Effects augmented reality (AR) platform, which lets artists and developers create AR content that can be added to photos and videos being recorded within the camera features built into Facebook apps, provides another monetization option. Snap Inc. (SNAP - Get Report) does brisk business selling sponsored lenses and geofilters that users can add to photos and videos, and Camera Effects opens the door for Facebook to do something similar.

In recent earnings calls, Zuckerberg has talked about how the process of monetizing Facebook's messaging apps would be one in which user-to-user interactions paved the way for organic interactions with brands and businesses, which in turn paved the way for "sponsored" interactions. This process has already played out in full on Instagram, and should gradually play out on both Messenger and WhatsApp, even if one is a little further along than the other.

Considering the kind of ad revenue that apps like WeChat, Line and Snapchat are able to pull in from their user bases, Messenger and WhatsApp still feel like underappreciated parts of Facebook's empire. Even after its recent selloff, Snap is valued at $100 per DAU. Though it's tough to justify that kind of valuation for Messenger and WhatsApp, since much larger portions of their user bases come from emerging markets, even a valuation of $30 per MAU would mean the apps have a combined value north of $72 billion.

Slowly but steadily, Facebook is showing how its messaging apps could justify such valuations, if not something greater, by becoming vital parts of its ad empire.

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