There are times when the text format for protocol buffers can be very useful. For example, you could use the text version during development if you wanted human-readability.

If I was pulling some information from Tumblr, I would at least read the introductory API docs at http://www.tumblr.com/docs/api (if you didn't do that, how would you even know what URL to hit?). If they were using Protocol Buffers, they would publish their .proto file on that page. In fact that page reads very much like an informally-specified .proto file; it contains a list of all the parameters and what they all mean.