Yahoo! has also been accused of "killing" Flickr, the image-sharing service it bought in 2005, by failing to develop foster innovation as Facebook and Instagram emerged to dominate photography online.
“We will operate Tumblr independently. David Karp will remain CEO. The product roadmap, their team, their wit and irreverence will all remain the same as will their mission to empower creators to make their best work and get it in front of the audience they deserve,” she added.
The all-cash sale will net 26-year-old New Yorker David Karp, who created Tumblr six years ago after dropping out of high school at 15, up to $250m.
Mr Karp said Yahoo! would help Tumblr get better and grow faster. It currently gets 300 million visitors per month who post 900 short pieces of text, photographs and videos per second.