Tumblr getting bigger and bigger is as surprising as Twitter getting larger or Facebook amassing even more of your photos. Which is to say, it's not.
To give you an idea of just how big Tumblr has become, in just a few years, the company brags that it's now part of the top 10 US sites, as measured by Quantcast.
The site now has some 170 million monthly visitors, worldwide. They're getting close to 18 billion pages every month, or more than 100 pages per visitor.
That's hardly surprising with 82 million blogs and almost 37 billion posts to date. Tumblr's format encourages reposting interesting stuff and page views.
While 170 million visitors is impressive, Instagram has some 100 million users at this point with its mobile only app.
Granted, registered users is not the same as active users, but it shows the importance of mobile, something Tumblr is well aware of.