As AVI pointed out, one reason conspiracy theories are popular is because conspiracies are common--but most conspiracies conflict and fail. And as Twain pointed out, “If you don't read the newspaper, you're uninformed. If you read the newspaper, you're mis-informed.” A friend of mine told me this morning that yesterday he'd been flying spotter for a large multi-site drug ring bust, of which not a word has hit the news. Maybe there are good reasons, but after a moment's thought we find that the newspaper/radio/TV fails to mention many other things we know about and think significant (what was that fire I saw on the way back from work?).
The fact that the newspaper can only print a finite number of articles (and can't hire enough reporters to report even on all the fires) must always be a little more abstract than the easily visible: "there was blood all over the road and not a peep in the paper." Plus, though it is always easier to see when you don't entirely share the political philosophy of the news source, quite a few of them seem to have unashamedly reverted to the old model of newspaper as political party organ. (What I've seen of TV news is worse; radio news is generally severely truncated.)
It's no wonder that there's a huge appetite for "alternative news." Some of the "alternative news" sources are aggregators with clear links to where the stories are from: Drudge Report is probably the most famous, with nothing but headlines and links and a picture or three. Even those headlines and pictures tell stories sometimes, and some things get left out--and you can infer some things about Drudge from that. Aside from the obvious fact that he likes a zingy headline better than a perfectly accurate one. Rantburg is a much more specialized aggregator, and includes a lot more commentary. It has a point of view too, but like Drudge you can follow the links to the Iranian or New Delhi newspapers and evaluate their credibility yourself.
There's the rub, I suppose. Even if you are of a disposition to learn more and try to understand, and not just to circulate "Oh wow that's deep" quotes allegedly from Neil Tyson, what do you bring to the task of evaluation?
To questions about "chem trails" I bring an education that tells me jet fuel, like gasoline, is a hydrocarbon and will produce water vapor that condenses in the cold just like my breath. I know it is mighty cold up there because I've been up there too, and I know gasoline burns hot enough to destroy simple additives. My understanding of human nature tells me that if 20,000 airplane mechanics have had their hands on the engines at least one of them would have betrayed the secret. When I put it all together it seems pretty airtight.
But suppose someone comes to the issue with one or more of those links missing, and in its place a rock-solid understanding that the birth rate is down and that high officials for years have warned of overpopulation, and that there are chemicals that prevent or abort pregnancy, and that organizations will buy what good publicity they can. In his analysis someone is obviously spraying birth control drugs all over the county, and since some people will object the perps must keep it secret and hire others to defend them. Since I would be defending the un-named perps, I must be one of the employees, and therefore I have no credibility.
Or on a less obviously crazy subject, Albert will find a story mentioning Palestinian suffering not very credible because "They brought it on themselves", while Bert will find a story mentioning Israeli suffering not credible for the same reason.
Or again: BenLaden supposedly had some erotic videos in his quarters. Do you find that story credible? I don't: even in hiding he was too exposed to risk that kind of embarrassment. It wouldn't be the first time someone was attracted and repelled by the same thing, but if he cheesed off one wife she could wreck his standing in minutes.
Of course most of the people who circulate nonsense never bother to go back to sources, and rely entirely on the credibility of their friends (who they know are credible because they agree with them) and the obvious fact that the story must be accurate because somebody made a nice-looking JPEG summarizing it.