Yahoo CEO Marissa Mayer danced on a tightrope this morning as she tried to explain the logic behind her company’s $1.1B agreement to buy social network site Tumblr. She told analysts in a conference call that the companies will work together, but separately. They appeal to different audiences, but she says they can complement each other. And since Yahoo is “all about brands” and advertising, it can gin up sales on Tumblr even though it has a history of being ad averse — and tolerant of porn. “We need to have good tools for targeting” ad messages, Mayer says. While it’s important for Tumblr to be “true to their voice,” the companies can “monetize [it] in a way that’s tasteful” while showing advertisers “the benefits of [Tumblr's young] demographics and the huge volume of users and traffic.” Tumblr’s 26-year-old founder and CEO David Karp — who Mayer called “one of the most inspiring entrepreneurs I’ve ever met” — underscored the cultural differences in a blog post this morning. Promising that “We’re not turning purple,” he ended his brief news announcement saying “fuck yeah.” Mayer vowed to “let Tumblr be Tumblr” operating “under the Tumblr brand and David’s vision” from its base in New York. Yahoo will help with the infrastructure, but won’t put its brand on Tumblr’s site. The connection “will be largely invisible to users.” She says it shouldn’t be hard to gin up ad sales at the social network, which just began to accept them a year ago. “Of the top 10 Hollywood studios, all use Tumblr to promote movies,” she says. “Tumblr views itself as a home for brands.” For example, Yahoo could “work with [Tumblr] bloggers who want ads.” She acknowledges that the businesses have different psychographics — Yahoo audiences are older than those at Tumblr where the average user is 25. “I would expect any ad units that we create [there] would be native and follow the form and function” of the site, Mayer says.

Wall Street appears divided on whether Mayer’s bold deal — the biggest since she took charge in July — will pay off. The stock is up just 0.5% in early trading. If Yahoo plans to leave Tumblr alone then “what is the value creation rationale for the acquisition?” asks Bernstein Research’s Carols Kirjner. “With the core [ad sales] business still in the throes of a turn around, we think this is too large of an acquisition, with too uncertain results.” But Cowen & Co’s John Blackledge says Yahoo has more than enough cash and Tumblr would “infuse new talent and a fast growing, hip blogging platform into the company.” And Barclays’ Anthony DiClemente says that the move “shows the willingness of Yahoo’s management team to be proactive in making moves that can bolster the company’s user engagement.” Still, he warns that Yahoo “needs to be careful about the manner in which it monetizes Tumblr, as a significant ad load and the perception of a large corporate owner could potentially alienate Tumblr’s core user base.”

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