Direct link to source: http://thenextweb.com/twitter/2012/08/22/tumblr-becomes-next...

I wonder if they tried to negotiate behind the scenes, to get Tumblr to pay $$$ for that access, and couldn't come to an agreement. Or if this is part of a negotiating strategy. Charging for third-party access seems logical given that they referred to their follow graph's "great value" when they shut off Instagram. Simply shutting off access at any price, on the other hand, doesn't make sense.