To be fair, many (most?) mainstream journalists are extremely active on Twitter.

Twitter is not that popular. The problem is that mainstream journalists spend too much time on it and confuse it for the real world. How many "news" stories are just repackaged tweets? A lot.

It became the formal gathering place once Twitter bent the knee and started handing out blue checkmarks like candy to every low-level media personality basically on-demand.

At some media companies, getting you your blue checkmark is part of the HR on-boarding process, ffs.

Journalists are clearly the heaviest, most important Twitter users at this point.

Twitter is where the professionalism in the mainstream media goes to die. Between headlines you’re likely either getting insider baseball, a circlejerk or a crowbar in the face with “mainstream” written on the side in sharpie. If all you have are the headlines, then you’ve chosen Twitter to be your alternative “RSS” client. A valid choice for most and not one I would criticize either, but not entirely what it is or “should” be.