They don't know, they will soon enough.  They think hundreds of thousands of people are reading their materials.  If they knew the true numbers, they wouldn't post there because the association with the site is going to be toxic for them.

In Olbermann's case a staffer posts his special comment a couple of hours before the show.  Olbermann posts at Daily Kos as free advertisement for his special comments.  Behind the scenes, Keith Olbermann and his show hate Daily Kos.  I'll explain that in a separate diary.

Merkley and Miller are politicians.  They are in Blue areas and Merkley has six years before he runs again. 

So here are your statistics for the 3 diarists who posted at Daily Kos yesterday as of this moment according to Jotter.

Olbermann - 2848 viewed your diary, 313 commenters.
Merkley - 1266 viewed, 129 commenters.
Miller - 2781 viewed, 141 commenters.

So there you have it at dying Daily Kos.  Three big names and not a single one is read by 3000 people.  I can get  ten times that much by writing a letter to the editor.