Yahoo officially acquires Tumblr for $1.1 billion, promises “not to screw it up”

Yahoo (s YHOO) and Tumblr announced Monday morning that Yahoo has officially acquired Tumblr for $1.1 billion in cash.

In the release, the companies noted that “Per the agreement and our promise not to screw it up, Tumblr will be independently operated as a separate business. David Karp will remain CEO. The product, service and brand will continue to be defined and developed separately with the same Tumblr irreverence, wit, and commitment to empower creators.”

Yahoo CEO Marissa Mayer also announced the acquisition on her own Tumblr, while Tumblr CEO David Karp wrote about it on Tumblr’s staff blog.

Tumblr has over 300 million monthly unique visitors, according to the release. (comScore had pegged the site’s April traffic at 124 million uniques.) The companies say that half of Tumblr’s users use its mobile app, and reiterated one of the reasons that Yahoo was willing to shell out over a billion dollars for a company whose revenues were less than $15 million last year: “The combination of Tumblr+Yahoo is expected to grow Yahoo’s audience by 50 percent to more than a billion monthly visitors, and to grow traffic by approximately 20 percent.”

“Our team isn’t changing. Our roadmap isn’t changing. And our mission — to empower creators to make their best work and get it in front of the audience they deserve — certainly isn’t changing,” Tumblr CEO David Karp said in a statement. “But we’re elated to have the support of Yahoo and their team who share our dream to make the internet the ultimate creative canvas. Tumblr gets better faster with more resources to draw from.”

“Tumblr is redefining creative expression online,” Yahoo’s Mayer said. “On many levels, Tumblr and Yahoo couldn’t be more different, but, at the same time, they couldn’t be more complementary. Yahoo is the Internet’s original media network. Tumblr is the Internet’s fastest-growing media frenzy. Both companies are homes for brands — established and emerging. And, fundamentally, Tumblr and Yahoo! are both all about users, design, and finding surprise and inspiration amidst the everyday.”

Yahoo and Tumblr are holding a conference call at 9 AM ET and we will be on the call.