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This scares many Tumblr users who fear the same thing could happen to their preferred social network.

“If Yahoo! wants to be cool again it could just drop the exclamation point,” Tumblr user @jamiemottram wrote. Another Tumblr user, @nopelarry, warned: “bye never signing into tumblr again.” A third said “it’s been real good knowing you, Tumblr.”

But tech analysts say Yahoo won’t necessarily repeat past mistakes because Ms. Mayer is new to the company and will take a completely different approach than past CEOs.

“That was the old Yahoo,” Mr. Kagan said. “I wouldn’t say that would happen now. Yahoo is a different company now that Marissa is running the show. So it’s quite possible that this could be successful.”

Mr. Solis also voiced confidence in Ms. Mayer’s ability to get the job done.

“There’s this skepticism that Yahoo’s going to do the same thing with Tumblr — take this cool brand and make it uncool,” he added. “But Marissa knows what she’s doing.”