As with Instagram, the change does not seem to have affected sharing outwards, but will impede users’ ability to follow friends that they may know from Twitter. This is part of Twitter turning the screws on sharing information about the users of its network out to other services like Instagram (and by extension, Facebook) and Tumblr.
First, a huge disclaimer: I worked at Tumblr for its first four years, and I’m still a shareholder. But I had no knowledge of this issue before reading this article, and I haven’t asked anyone at Tumblr about it.
That said, purely from the outside, this looks like some serious bullshit on Twitter’s part. When they cut Instagram off, the between-the-lines explanation was because of Twitter’s rocky history with Facebook, with Facebook’s previous blocking of Twitter. It was a dick move to everyone using Instagram, but I guess Twitter’s rationale was “two dick moves make a right.”
I’m not aware of any such history with Tumblr, so I can’t think of any reasonable explanation for Twitter’s motives here other than the obvious one: Twitter will now only permit large services to add value to Twitter, not get any value from it.
And Tumblr was (is?) even working with Twitter to be a major Cards partner.
So when Twitter says something like “[developers] will need to work with us” to “identify areas of value” when they get big enough for Twitter to notice, I don’t have high hopes for what “working with us” might actually entail.
This is how Twitter treats its “partners”.