Tumblr is redefining creative expression online,"the Yahoo chief executive, Marissa Mayer, said in a statement. "On many level, 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."

The deal, which is expected to complete in the second half of the year, is almost all cash, is equal in value to Yahoo's money in hand at the end of its first quarter in March -- but it has substantial extra assets in short-term securities, shares in the Chinese site Alibaba, and other equities which it can liquidate.

Yahoo promised "not to screw it up" as it formally announced its acquisition of the Tumblr blogging platform for 1.1 billion dollars ahead of the US stock market opening on Monday.

Yahoo plans to use its personalization and search technologies to tie Tumblr users more tightly into its network -- thus dragging their attention away from rival social networks.

Announcing the deal on Tumblr's staff blog, David Karp, Tumblr's 26-year-old founder, said: "We're not turning purple. Our headquarters isn't moving. Our team isn't changing." Karp will remain Tumblr's chief executive. The deal will see Karp become the latest high-school dropout turned multimillionaire. Thank you.