Reporters aren’t scientific. They could use a site like this one, if they happened to know about it. But they’d know this is not really representative of the population. Perhaps a special segment of it. I heard a reporter on the radio this morning reporting on the reactions of folks in West Virginia. He had comments from a lawyer and someone else with a fairly decent job. Talk about representative! that’s sarcasm; I don’t know what his point was
Sometimes—and they’ll say “a commenter Z on X site said Y”. But it is usually looked down upon and frowned upon. Then they take pot shots at each other for having done it.
I suspect that they use sites more like the ones below to track public opinion. There’s really no way of knowing what district anyone online is in and that is what is most important to them.
A handful of reporters, from NYTimes, HuffingtonPost, etc. have used fluther for anecdotal opinions and quotes. I doubt it would be useful for any statistics though.