Image source: TwitterCopyright Aaron Durand (@everydaydude) for Twitter, Inc. 

During Twitter's (NYSE:TWTR) second-quarter earnings call, CFO Anthony Noto told analysts that its upcoming slowdown in revenue growth is the effect of moving from two growth drivers to one. "We've transitioned more to revenue being driven by average revenue per advertiser," Mr. Noto told listeners.

A recent RBC Capital survey done in partnership with Advertising Age indicates things aren't going so well: 28% of the advertising professionals surveyed said they plan to decrease the amount they spend on Twitter; 26% said they plan to increase their Twitter ad spend. That's the first time RBC and Ad Age have seen the results skew negatively for Twitter.

Missing the shift

Over the past few years, advertisers have spent a larger portion of their ad budgets on digital advertisements. In fact, digital ad spending in the U.S. is expected to surpass television advertising for the first time ever this year, according to eMarketer.

Twitter's plan to capture those ad dollars is to focus on video. It bought Periscope last year, and it started snatching up the rights to big events like Thursday Night Football and the U.S. presidential debates. In management's second-quarter letter to shareholders, it says mobile video advertising is a "large incremental market opportunit[y] with strong growth."

To be sure, mobile video ad budgets are growing, but Facebook (NASDAQ:FB) and Alphabet (NASDAQ:GOOG) (NASDAQ:GOOGL) unit Google are the big winners.

YouTube's mobile engagement continues to climb. Google CEO Sundar Pichai told analysts that "every indication we see is that the growth is very, very strong, being driven by mobile" during the company's second-quarter earnings call. Alphabet CFO Ruth Porat told analysts that YouTube revenue is growing at a "very significant rate, driven primarily by video advertising."

Facebook, meanwhile, is seeing users spend more time watching videos on both Instagram and its flagship platform. Of those surveyed by RBC and Ad Age, 69% were positively inclined toward Facebook's autoplay video ads. Facebook recently revealed an error in how it reports average view time for video ads, but importantly, that has no impact on advertisers' return on investment.

Return on investment is one of the most important metrics

Marketers are going to spend money wherever they can get the most bang for their buck. Facebook is already highly rated among advertisers for its return on investment, and 60% of marketers believe their ROI has improved on Facebook over the last six months. That number is 42% for Google, 29% for YouTube, and just 25% for Twitter; 21% of advertisers believe their ROI has decreased on Twitter over the last six months.

During the company's second-quarter earnings call, Twitter's management noted that its ad prices are relatively high compared to other social networks like Instagram. "We do think that while we have a premium, that premium is justified overall," COO Adam Bain told analysts. Twitter's customers -- that is, its advertisers -- seem to disagree.

As long as advertisers aren't seeing as good a return on investment compared to other platforms, Twitter will be unable to grab a larger share of their budgets.

Twitter isn't increasing the number of total advertisers on its platform, it's not increasing the number of ads its users see -- by increasing either time spent per user or ad load -- and it's not increasing the average amount its advertisers spend on the platform. All those add up to no ad revenue growth, which is evident in the company's third-quarter outlook. It expects just $590 million to $610 million in revenue for the third quarter. That represents no growth sequentially from the second quarter, and just 5% growth year over year.

As marketers' sentiment toward Twitter continues to decline, it might not be long before we see negative growth in Twitter's ad revenue.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.