Depending on how you count, Twitter (NYSE:TWTR) added just 3 million or 4 million net new users last quarter. Excluding SMS Fast Followers, which don't see advertisements, Twitter has added just 5 million users over the past six months to bring its total to 307 million. That's up just 8% year over year. Even Facebook (NASDAQ:FB), and its massive user base of 1.49 billion, is growing faster than that, increasing 13% year over year in the second quarter.

But Twitter says it has hundreds of millions of more people coming to its website -- its homepage, search results, individual profiles -- that it hasn't yet monetized. During the third-quarter conference call, newly appointed COO Adam Bain said the company will take the first steps toward monetizing those users. On the call, Bain told investors the company will "begin to run a pilot here in Q4 for on-Twitter logged-out monetization."

Twitter
Twitter's logged-out homepage. Source: Twitter.

Growing revenue without user growth
Without significant user growth, Twitter is being forced to look elsewhere to grow its ad revenue. CFO Anthony Noto has said there's room to increase its ad load up to three times its current rate, but the company seems hesitant to do so. Last quarter, ad revenue increased 13% sequentially -- 1% from user growth, and 12% from increased ad prices and engagements. The latter factor benefited significantly from auto-play video ads, according to Noto.

But investors can't expect a 12% bump from new ad features every quarter. Facebook increased its ad revenue per user just 7.8% sequentially from the second quarter to the third quarter last year, for reference. With analysts expecting 42% revenue growth in 2016, Twitter must find a way to capitalize on its wider audience, which it recently told developers is more than 1 billion people.

Earlier this year, Twitter started inserting promoted tweets into syndicated feeds such as those on Flipboard. Those ad units have failed to catch on with many other publishers, however, and they leave revenue growth out of Twitter's control.

The only viable options Twitter has left is to either raise its ad load or start showing ads to people who visit its homepage or user profiles and land on a search-results page.

Setting the stage
Last quarter, Twitter expanded ads to its MoPub publishing partners, showing promoted tweets, videos, and app-install ads across the 1 billion devices MoPub reaches. Some of the people seeing those ads included people who didn't use Twitter. Reaching those people with promoted tweets has provided some valuable insights to Twitter. "We're going to take some of [those] learnings," Bain told investors on the third-quarter earnings call, "and apply it back on Twitter logged-out ad products."

This quarter, Twitter will only be running a pilot of its logged-out ad experience, meaning just a fraction of visitors without Twitter accounts will see ads. It's important for Twitter to get the ads right to maximize the potential revenue from these visitors. Non-users are significantly harder to monetize than active users, because Twitter has a limited set of information about each of them. It can determine location, the website they last visited, and whatever context it can pick up from clicks on Twitter.com.

The new Moments feature offers an opportunity to present some of the best Twitter has to offer to logged-out visitors. It's also an advertising opportunity, and probably its best for non-users. Aside from the Promoted Moments advertisers can buy, Moments presents an opportunity to advertise against an implicit interest based on whatever the Moment is about. Twitter can sell ads against Moments just like cable TV networks sell their ad space.

The opportunity
Twitter believes its opportunity to monetize logged-out visitors is huge. Overall, it expects it can generate ad revenue at half the rate of its logged-in users. That's extremely optimistic, but Bain told analysts, "What we've learned so far doesn't allow us to deviate from what we said previously in terms of what we think the opportunity exists in terms of logged-out users."

If that's truly the case, Twitter could expect to nearly double its ad revenue from the additional monetizing of logged-out visitors. Investors shouldn't expect that revenue to show up overnight, of course. Remember, this is just a pilot. Perhaps we can find out more about the results of the pilot when Twitter reports its fourth-quarter results and see if the lofty monetization rate assumptions hold up. In the meantime, investors should stay poised for news about the start of the pilot program.

Adam Levy 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.