Since Tumblr has supported multimedia content for a while, I'd say no. Twitter was (and is still mostly) text only. Protocols don't have to be text-only, but that's what makes Twitter feel like one, I guess? Just my two cents.
To my mind the fact that including pictures or video is less ad-hoc with Tumblr makes it more of a protocol, not less. Honestly Twitter doesn't feel particularly special to me - Facebook, Google+, even Livejournal feel like they're the same "kind of thing". Heck, at times Youtube behaves the same way. But I never got hugely into twitter, maybe it's different if you're a real fan.
>To my mind the fact that including pictures or video is less ad-hoc with Tumblr makes it more of a protocol,
I see what you mean, it's like Twitter but customizable to the core - true to any good protocol. What made twitter really a protocol was that so many clients were made for its liberal API.
