It may not be bigger worldwide, but Alexa shows Pinterest is now ahead of Tumblr in overall US traffic and reach. On the list of top 500 sites in the United States, Pinterest weighs in at #17, one ahead of Tumblr.
The photo sharing site shares a little with Tumblr in function, but the form of the site is completely different. Tumblr combines microblogging, photosharing, and GIF-heavy offbeat content into a full blogging platform that rivals WordPress and Blogger for adoption. Pinterest, on the other hand, has a more modern interface that displays a feed in an adaptive format, giving users the ability to scan through dozens of pictures at a time without much scrolling.