The Yahoo logo outside of its offices in Santa Clara, Calif.
The Yahoo logo outside of its offices in Santa Clara, Calif. (Associated Press file)

SAN FRANCISCO -- Yahoo is buying online blogging forum Tumblr for $1.1 billion as CEO Marissa Mayer tries to rejuvenate an Internet pioneer that had fallen behind the times.

The deal announced Monday is Mayer's boldest move since she left Google 10 months ago to lead Yahoo's latest comeback attempt. It marks Yahoo's most expensive acquisition since the Sunnyvale, Calif., company bought online search engine Overture a decade ago for $1.3 billion in cash and stock.

Yahoo is paying mostly cash for Tumblr, dipping into what remains of a $7.6 billion windfall reaped last year from selling about half of its stake in Chinese Internet company Alibaba Holdings Group. Taking over Tumblr will devour about one-fifth of the $5.4 billion in cash that Yahoo had in its accounts at the end of March.

While hailing Tumblr as a fount of creativity that attracts 300 million visitors each month, Mayer told analysts Monday that she is "making a sincere promise to not screw it up." David Karp, a high school dropout who started Tumblr six years ago, will remain in control of the service in an effort to retain the same "irreverence, wit and commitment to empower creators," Yahoo said.

Tumblr, which will remain based in New York, has about 175 employees while Yahoo has 11,300 workers.

In his statement, Karp predicted Yahoo would help Tumblr grow even faster as he strives "to make the Internet the ultimate creative canvas. "

The deal is expected to close during the second half of this year.