If you look at the 'Frequency of social media site use' graph though Twitter is far more popular on a daily basis than Pinterest. This seems a more significant statistic.
http://tctechcrunch2011.files.wordpress.com/2013/12/screen-s...
http://tctechcrunch2011.files.wordpress.com/2013/12/screen-s...
This theory is backed up by http://www.alexa.com/topsites/global;0 where Twitter is 12th and Pinterest is 30th.
Of course, popularity doesn't pay the bills!