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(CNN) – Yahoo announced a $1.1 billion deal for blogging site Tumblr early Monday, and said it “promises not to screw it up.”

The deal, which had been rumored since last Thursday, will help Yahoo to tap into the younger, active online user base at Tumblr. But the deal raises concerns among some Tumblr fans that the site could end up being cluttered by the ads that brings in billions of dollars a year to Yahoo. Tumblr CEO and founder David Karp has resisted the use of traditional display advertising on Tumblr to this point.

Yahoo-TumblrTo answer those worries, Yahoo said that Tumblr will be independently operated as a separate business with Karp staying on as CEO. Yahoo CEO Marissa Mayer posted her own Tumblr post to try to assure fans of the site they need not panic. In a call with investors and analysts Monday, Mayer referred to the deal as a “game changer” for Yahoo. She said there will be no Yahoo branding on Tumblr.

Mayer compared the deal to what she said were the most successful tech acquisitions in the past, Google’s purchase of YouTube and the purchase of PayPal by eBay (EBAY, Fortune 500), combinations that she said were virtually invisible to users of the Web sites that were being acquired.

But while she sought to assure Tumblr bloggers and readers that the site will stay the way they like it, she also promised analysts that the deal will give Yahoo a chance to “monetize” Tumblr in a way that “is meaningful … to the user experience.” She said that Karp and his team agree that Tumblr has now grown up to the point where it makes sense to sell more ads.

Karp, who was not on the call, also sought to assure Tumblr bloggers.

“Our team isn’t changing. Our roadmap isn’t changing,” said Karp in a statement. “Tumblr gets better faster with more resources to draw from.”