The exit of Yahoo's (YHOO) No. 2 executive signals an abrupt end to Marissa Mayer's honeymoon.

Eighteen months into her tenure -- a long time in the Internet world -- the Mayer/Yahoo turnaround hasn't happened.

Advertising, Yahoo's chief way of making money, is still not growing. That is likely to be more apparent next week when Yahoo reports its fourth-quarter earnings.

I hate to point this out because I want Mayer to succeed and turn around this great valley institution, one of the first Internet companies that embodied the fun, almost-anything-goes ethos of Silicon Valley during the early days of the dot-com boom.

And I am intrigued by Mayer, who was one of the highest-ranking women at Google (GOOG) and is now one of tech's most prominent female CEOs.

We were given a blessed reprieve from thinking about Yahoo's core problems when Mayer was named the company's fifth chief executive in five years.


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She brought a sense of excitement and dynamism that the company had been missing. She tweaked the company's logo and went on a startup shopping spree, including paying $1.1 billion for Tumblr, the social blogging platform, her highest-profile deal to date. She addressed employee morale by handing out iPhones and serving up free food. She tried to rein in Yahoo's flabbiness by pulling in its scattered workforce and requiring people to show up at the office.

And on the content side, the company has succeeded in attracting more Web traffic. According to comScore, when it comes to desktop traffic, Yahoo had more visitors in December than Google, Microsoft and Facebook.

This is helpful, but not enough. The company needs to attract big audiences that advertisers know they can reach. Right now, much of its products feel scattershot. Other than Yahoo groups, there isn't a lot that Yahoo offers that I can't live without.

Masking some of its underlying problems, Yahoo's stock has been on a tear, up more than 150 percent since Mayer arrived in July 2012.

But make no mistake. The stock market excitement over Yahoo, in my guestimation, has one-third to do with Mayer and two-thirds to do with what are known as Yahoo's "noncore assets," chiefly its 23 percent stake in Chinese Internet firm Alibaba, which is expected to go public in the next two years. Yahoo sold 40 percent of its Alibaba stake in 2012 for $7.6 billion and can continue to ride the Alibaba wave for a long time.

But eventually, even that money will be gone.

And that will leave its primary core asset: content that produces online advertising. Last week's exit of Henrique de Castro, Yahoo's chief operating officer who was hired by Mayer from Google, cast a harsh light on what is happening there.

According to eMarketer, Yahoo's U.S. digital ad market share in 2013 fell to 5.8 percent, down from 6.8 percent in 2012. The company reported that its third-quarter revenue was $1.1 billion, mostly from advertising. That was down 5 percent from a year ago.

"Yahoo is losing market share in digital advertising, but especially in display advertising, which is in general a growing market," said Karsten Weide, program vice president at IDC. "They are also getting weaker in absolute terms. They are selling less. They are making less."

After 18 months on the job, she hasn't cracked Yahoo's central problem, which industry analysts say is that it's hard for advertisers to reach Yahoo's users.

"This is a company whose users are fragmented across a wide array of sites, like Tumblr, sports, Flickr, mail, home page," said Clark Fredricksen, a vice president at eMarketer. "For an advertiser, that means it's not so easy to reach users at scale."

Yahoo has also been behind on the shift of users to mobile. "You have to have the means to put yourself in front of consumers faces," said IDC's Weide. "You have to have apps preinstalled on mobile tablets and phones."

At Yahoo, "little of that is happening," he said.

At the Consumer Electronics Show earlier this month, Mayer did announce some new ad products, but analysts say it's too early to tell if these will make a difference,

Perhaps Yahoo's biggest problem is the strength of its competitors -- Facebook, Google and Twitter.

That means one thing. Focus. Mayer has dazzled the tech world with glamour: Vogue shoots, Katie Couric and all the rest. Now she has to dazzle in one unglamorous and difficult way -- turning Yahoo's advertising story around.

Contact Michelle Quinn at 510-394-4196 and mquinn@mercurynews.com. Follow her at Twitter.com/michellequinn.