Very interesting graphic depicting social media average minutes per visitor. This is a much better metric than number of registered users.
I’m sure Tumblr and Pinterest are rapidly growing in both visitors and time per user since both of the platforms allow for easy content creation and compilation.
Update: californiachillwave makes a good point. Pinterest is catching on fire because it is easier to organize other peoples thoughts into your own profile. In my opinion, Tumblr is a blogging platform so you share content keeping your followers in mind.
Source: The Wall Street Journal