[derefr]

There is no single forum that contains as participants all of academia (and nobody else), with public viewership and the ability to DM the participants.

(One might say that Usenet did, at one point, contain all of academia within a few of its hierarchy-roots; but there's nothing stopping random quacks from posting to those. Imagine a version of the 80s where Usenet's sci. hierarchy would only accept posts from actual science departments in universities and research institutions—but where it was still mirrored for reading everywhere else—and you'd be closer.)

Really, what I badly re-invented here, was scientific journals, but with a 140-character publication limit ;)

Or IRC channels in +v mode, but where there's only 10 of them, and each one has millions of unvoiced members and thousands of voiced members.