What you hear informs your world-view.
When everyone consumed the same (mass-)media, everyone had a fairly consistent (if sometimes wrong) world-view. It wasn't always correct, but at least everyone was on the same page and talking the same language.
Now aggregators, social news sites and predictive "you might also like" functionality has made it ever-easier to only see things you agree with, and to never even be aware of events and attitudes you don't.
This gradually, subconsciously leads you to believe that "everyone" thinks the way you do (so anyone who disagrees is obviously a kook and can be ignored), and to confuse "commonly-held beliefs in your particular subculture" with "proven, empirical facts" (or if you prefer, just "commonly-held beliefs for the entire population" ;-).
(I believe that this on its own is a major cause for the worsening "culture war" in the US, as well as the rise of tacky smear-based campaigning and staggering bitchiness of modern politics.)
This situation was always a possibility (witness the people who only ever watch Fox News, for example), but at least by deliberately limiting themselves to only one source it was obvious (often even to themselves) these people were closed-minded and willingly ignorant.
They might only watch one channel and forswear all other viewpoints, but the majority of people knew what they were doing and why it was dumb... at some level even them.
Now, with the advent of the internet and the massive decentralisation of news, discussion and rhetoric, hundreds of blogs and publishing outfits have sprung up for each mindset, niche political leaning, sexual preference, subculture... you name it.
This means that it's entirely possible to only read tens or hundreds of different sources... and yet still only really hear what you want to hear.
And yet, because you're reading so many different, unconnected sources the agenda they're pushing looks even more reasonable and widely-believed than when it could be easily written off by others as the agenda of just one channel, or just one new-corporation owner.
So although in this brave new world of millions of dissenting and conflicting voices the truth can be found more easily than in the monolithic, old world of Big Media... it's also made the truth harder to spot when you do find it, and it's made the bullshit look a lot more widely-supported and convincing than ever before.
I'm still not sure what to do about this state of affairs - I'm working on a few ideas which might help in the long run, but it's a serious problem that in the short term leads to social paralysis and all the he-said-she-said unconstructive, name-calling bitchiness of the politics of the last few years.