Activist investors are awesome at concern-trolling. Just look at Starboard Value's open letter to Marissa Mayer, a new contender for the underminer's hall of fame. The hedge fund "wants Mayer to stop buying companies" because it's worried recent bets haven't paid off.

Without even naming Tumblr specifically, Starboard incited analysts to call Tumblr a flop, reports Fortune:

"Tumblr is relatively inconsequential," said Ben Schachter, a long-time Yahoo analyst at Macquarie Securities. "They continue to lose share of dollars and mindshare. They haven't been able to execute a turnaround. A lot of the new strategy is the same as the old strategy and it's not working yet."

Schachter said Tumblr had approximately $10 million to $15 million in revenue at the time of the acquisition, and since Yahoo has not updated the numbers, revenue has likely not grown by much. Even if Tumblr revenue had grown, say, twice as fast as traffic, it would have added only single-digit millions to the company's revenues.

Mark Mahaney, another longtime Yahoo analyst, told Fortune the $1.1 billion deal to buy cool and attract other wunderkind founders was "inconsequential."

Tumblr was cagey about its numbers when it was private and that hasn't changed under Yahoo, but the company is standing by its Millennial man:

"Tumblr has been important from a consumer standpoint, an editorial standpoint, an advertising standpoint, and a technology standpoint," Kathy Savitt, Yahoo's chief marketing officer, said in an interview.

Traffic at Tumblr has grown by about a third to 400 million monthly users since last year's acquisition, and the numbers of blogs and posts have nearly doubled, according to the company's own figures. Savitt noted that while Tumblr operates independently, Yahoo and Tumblr have collaborated in productive ways. Yahoo's ad technology now powers Tumblr's "sponsored posts," which are the main vehicle for monetizing the microblogging platform. Tumblr-sponsored posts from brands like Lipton, Lexus, and Lionsgate appear on Yahoo. "Brands are able to create rich stories that are engaging in their own right," Savitt said. Yahoo has used the Tumblr platform for some of its own digital magazines.

Tumblr has allowed Yahoo to bring those magazines—which span categories such as technology, travel, food, style, beauty, and movies—to market faster, Savitt said. It also allowed Yahoo's editorial teams to produce richer content. With Tumblr, the company has been able to tap into a younger audience, which is important to Yahoo's future, Savitt said.

Savitt's defense doesn't inspire much confidence. But a Yahoo employee, who previously worked at Facebook, pointed out that the skepticism is premature. "Tumblr has lots of traffic and they can make ad dollars, but it's taken them much longer than expected for sure," said the source, adding: "How long has it [taken] Instagram to monetize? Still hasn't happened yet."

The difference between Tumblr with Facebook's $1 billion hipster purchase is that Instagram is currently no. 8 among free apps in the Apple App Store—and Mark Zuckerberg made sure activist investors couldn't tell him what to do.

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