Through the drama, Twitter’s advertising business has been a bright spot. Last year, under the well-respected COO Adam Bain, revenue grew 58%. In a February feature story about Twitter’s turnaround, I noted that many analysts believed that Twitter’s shares had hit the bottom, and that they could only go up. Only two of the 43 analysts covering Twitter had a sell rating on the stock. A Deutsche Bank analyst report from February sarcastically called Twitter “The Most Troubled 30%+ Compound Annual Growth Rate Company on Earth,” noting that the negative narrative around the company “feels a bit extreme.” The point being? Even if user growth had stalled, Twitter’s business was solid.

That view may have changed following the company latest quarter’s results on Tuesday. Twitter, which remains unprofitable, reported 36% in year-over-year revenue growth, at the bottom end of analyst expectations. More significantly, the company’s guidance for next quarter’s came in $88 million lower than analysts expected.

Investors were spooked: Twitter shares fell by 14% in after-hours trading.

On the company’s first quarter earnings call Tuesday, Bain blamed the Twitter’s revenue struggles on slower than expected “brand” spending.

CFO Anthony Noto pointed to potential macroeconomic headwinds (for example, ad spending in the restaurant category was weak) and noted that the entire Internet advertising category has not yet reported its earnings yet, implying that the digital advertising business had suffered overall this quarter. “The jury’s out as to how the whole category has shaped up across the board,” Noto said. (Yahoo and Google have already reported lukewarm earnings; only Facebook, which reports its quarterly earnings tomorrow, remains.)

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Beyond macro headwinds, Twitter has fully penetrated the U.S. ad market, which is Twitter’s largest market for revenue, Noto said. Now, the company must increase its “share of wallet,” or the amount of money advertisers allocate to Twitter versus other forms of advertising. “Penetration has run its course,” Noto said. “Some of slowdown is moving from two dimensions of growth for those advertisers to one dimension of growth,” meaning that Twitter can only grow by increasing spending from existing advertisers in the U.S., not from finding new ones.

Bain defended Twitter against negative sentiment in the press (a question posed during the call), saying that the company re-signed deals with three of the four big advertising holding companies, all of which increased spending by around 40% over last year. He added: “And those are video deals.”

Video, and Twitter’s success selling video ads, was a big theme for this call. Curiously, Bain referred to Twitter’s existing ad products, the “Promoted Tweets” and “Promoted Trends” that he has been hawking for the last five years, as “legacy” products. The reason? Users like video ads better than Promoted Tweets, Bain said. Promoted Tweets are old news, video is the future.

And about that user growth, the all-important metric that Wall Street has previously obsessed over: Twitter actually added five million new monthly active users last quarter to 310 million. But investors didn’t seem to care.