Tumblr, too, is doing exactly that, if you believe this January survey:
http://articles.latimes.com/2013/jan/11/business/la-fi-tn-tu...
Another survey, this one from December and including all Americans (not just teens), showed people spend more time on Tumblr than Twitter, Pinterest and LinkedIn combined:
http://www.marketingcharts.com/wp/interactive/in-december-am...
That seems to suggest Tumblr's worth more than Instagram.