Sunnyvale-based Yahoo! has officially announced that it is buying photo blogging site Tumblr, in a deal worth approximately $1.1 billion in cash. According to Yahoo, it has a promise "not to screw it up", and will allow Tumblr to be independently operated as a separate business, led by CEO David Karp. Tumblr is based in New York. Yahoo said that Tumblr now has more than 300 million monthly unique visitors and 120,000 signups every day.