I can imagine Twitter publishing a application/tweet+json media type or similar, and clients and servers either supporting this or not (and exchanging Accept headers and 415 Unsupported Media Types as they go), but I don't get what that has to do with HAL.

For example, the "comments" link relation in

http://developer.github.com/v3/pulls/

points to

https://api.github.com/octocat/Hello-World/issues/1/comments

but no-where in the document does it say what media types that endpoint will accept.