Yahoo's most important business actually shrunk for the first time this quarter

One of Yahoo's most important businesses under CEO Marissa Mayer has been the so-called "Mavens," short for mobile, video, native, and social.

Mayer likes to say that Mavens revenue was largely non-existent before her arrival in 2012, when most of Yahoo's revenue relied on the legacy desktop search and display advertising businesses. Under Mayer's watch, Mavens revenue grew into a $1.6 billion revenue business last year.

But Mavens revenue has continued to see its growth decelerate over the years, with its growth dropping to a mere 7% year-over-year in the first quarter of this year (versus 59% the previous year), the lowest ever.

And in Yahoo's second quarter earnings on Monday, Yahoo disclosed that Mavens saw negative growth for the first time. Excluding new accounting changes that went into effect this quarter due to a contract with Microsoft, Yahoo's Mavens revenue was at $385 million for the quarter, down roughly 4% from the same quarter of last year. (Under the new accounting method, Mavens revenue was $504 million, up 25% year-over-year). Mavens grew 60% in the same quarter of last year.

"Mavens is a key part of our strategy, our growth, and our turnaround story. And we will work very hard to get the Mavens collectively back to growth in the 2nd half of the year," Mayer acknowledged during Yahoo's earnings call on Tuesday.

Wall Street doesn't seem too impressed by it. Mizuho Securities' analyst Neil Doshi told Business Insider that it's a big "negative" as Mavens have been one of the few areas of Yahoo that showed future potential.

"I think it just goes on to show the struggle that Yahoo continues to have in terms of pivoting their business model from the desktop web, display company to a mobile native social platform," Doshi said. "The fact that Yahoo can't seem to show growth in that business i think goes to show the real challenge Yahoo has in their turnaround."

Of course, as Mavens continues to become a bigger part of Yahoo's overall revenue, it will be difficult to keep up the same pace of growth as it did in previous quarters. But Mavens still represented only 38% of the revenue (excluding accounting changes), and to go from 60% to negative 4% growth is a big slowdown for any business.