This is the 8th language that Tumblr supports, following the addition of support for Italian in February and Spanish in August. The others are English, German, French, Japanese and Turkish. Support for other languages is on the way.
Tumblr hit 10 billion posts last month (there are now more than 11.7 billion posts) and around 37.5 million posts are added to the service every day on almost 32.2 million blogs that are hosted on Tumblr. The company raised $85 million in a funding round last month at a reported $800 million valuation.
If you would like to start using Tumblr in Russian, you can do so by changing your language settings on the Preferences page.