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Is the concept of the ‘mainstream media’ a myth?
Yes and No. There really are mainstream media in this country. It's not a concept, it's a reality. You can talk about the mainstream media and make observations that are mostly true, today. The thing that is mythical about it, is exactly as Erasmussimo says. Those that usually invoke the 'mainstream media' as having this-or-that-bias, or this-or-that-agenda, or this-or-that-characteristic, talk about the media as they were 30 years ago. The mainstream media are more diverse, and more in number, but conversely owned by less entities today.
Here's an example of a mythical mainstream media concept:
"The mainstream media and their allies in the Democratic Party all feel put-upon." - Rush Limbaugh
Here's an example of a non-mythical mainstream media concept:
"News networks, you know, they have this idea that they're being objective. But news has never been objective. It's always 'Our top stories tonight.' That's a list … that's a subjective… some editor made a decision: 'Here's our top stories. #1: There's a fire in the Bronx. #2: They arrested Martha Stewart.'" - Jon Stewart
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If you believe it is a myth, why is such a myth perpetuated?
The mythical portion of the media myth is usually invoked in order to paint a picture of the media that supports the invoker's position. The problem with this is that the collective invokee (i.e. 'the mainstream media') includes Rush Limbaugh, Ann Coulter, Fox News Channel, the New York Times, dailykos.com and Michael Moore.
(As an aside, I'd just like to mention that only one entity in the preceding sentence consistently presents a diversity of political views. It is also the one which is invoked most often when referring to a "liberal media!" ...

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If you do not believe it is a myth, what are the core defining features of the ‘mainstream media’?
Nowadays, Fox News Channel is part of it. Michael Moore is part of it. Rush Limbaugh is part of it. I don't think many or any bloggers are a part of it. IMO, anything that reaches more than several million readers/viewers/listeners in every state of the US, is part of the USA's 'mainstream media.'
When Limbaugh speaks about the 'mainstream media' as "them," the unintended irony is astounding.