Compared, to say, $1b for instagram? (i still dont get that)
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.
I see Instagram as a defensive acquisition -- they don't want to lose market share to a competitor. It took Instagram only two years to reach 100 million users.