Given how negative the news flow surrounding Facebook (FB) - Get Report has been in recent months, it's easy to forget how many untapped and potentially large revenue streams it has available to it.

The social media giant's launch of a search ad test for its core search engine and the Facebook Marketplace platform acts as a fresh reminder of this fact. The test, which is taking place in the U.S. and Canada, involves showing Facebook's mainstay news feed ads within mobile search results, with the ads targeted based on the type of content a user is searching for. TechCrunch states "a small set of automotive, retail and e-commerce industry advertisers" are currently taking part.

Search underpins the most valuable ad business on the planet -- Alphabet's (GOOGL) - Get Report Google Search, which is estimated by research firm eMarketer to produce over $73 billion in revenue this year. And many online ad players that don't run general-purpose search engines, such as (AMZN) - Get Report , Yelp (YELP) - Get Report and Twitter (TWTR) - Get Report , have also had success running ads against searches done on their platforms.

While Mark Zuckerberg's company isn't in Google's league as a search platform, and some types of searches are much easier to sell ads for than others, Facebook did report seeing 2 billion searches per day back in July 2016, up from 1 billion in 2012. The number is most likely higher now -- and even if it isn't, effectively running targeted ads against just a fraction of 700 billion or so annual searches would represent a pretty valuable business.

The business would also, arguably, be a nice complement to Facebook's news feed ad business, which remains by far its largest revenue source but is getting hurt by usage pressures in North America and Europe. Whereas the news feed ad business relies heavily on Facebook's considerable user data, a search ad business could could lean heavily on search intent. And as Google or Amazon would vouch, search keywords can sometimes provide a better read on what a person is looking to spend money on at a particular moment than that person's demographic info or likes.

At the same time, a search ad business could (as signaled by Facebook's test) integrate with Facebook's existing ad-buying tools and support existing news feed ad posts. It could also leverage Facebook's advanced tools for measuring the payoff delivered by ads, and, when useful, leverage the company's user data to improve keyword-based targeting. And over time, it could be expanded to other platforms, such as Instagram and Messenger.

It remains to be seen just how serious Facebook is about creating a large search ad business. However, this definitely feels like a logical time to try and create one, given the pressures faced by the news feed ad business and the revenue opportunity it presents. And it's certainly far from the only potentially lucrative revenue stream that currently has a small or non-existent impact on Facebook's top line.

On the whole, the company's "stories" services and messaging apps remain lightly monetized for the time being. Facebook has recently begun to get serious about monetizing Instagram Stories, which has over 400 million daily active users (DAUs), but from all indications the platform's ad load is still much lower than that of Facebook or Instagram's news feeds. WhatsApp's stories service (it's known as Status, and has over 450 million DAUs) isn't yet monetized, and will begin to see ads in 2019.

Facebook also wants to monetize WhatsApp by charging businesses to message users in certain situations, and could conceivably try to do the same with Messenger, where it has begun showing ads to some users. And in spite of some early missteps, Facebook's Watch video platform could end up providing a lucrative source of video ad inventory.

All of this is worth keeping in mind as Facebook, which claims more than 2 billion daily users across its four main platforms, continues trading more than 30% below its July highs. The challenges faced by Facebook's core news feed ad business can't be ignored, nor can the potential of its recent controversies to distract management and hurt employee morale. However, the size of its many untapped and lightly-tapped revenue opportunities -- including opportunities that aren't impacted by core Facebook's pressures -- can't be ignored either.

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