Normally I think xkcd hits it out of the park, but today's contribution I think is way off target. The premise of the comic is that it costs far more money to run ads on every major political news site than it would to "pay five college students $20/hour to camp the site 24/7 and post the first few comments the moment a story goes up, giving you the last word on the subject and creating the illusion of consensus."
The alt-text proceeds to mock comment structures based on "voting" up good comments and down bad ones. But the real answer is "most commenters on major news sites are morons, and most readers of those sites have to be aware of that fact." If persistent internet trolling was enough to seriously change views, Ron Paul would be President-for-life.