I'm reading an excellent book right now called The Filter Bubble, by Eli Pariser. It's about why, if you and I enter the same search term into Google, we end up with different results.
What's riveting is how the 'personalization' of the web can be both helpful and harmful. Heaven knows that with the amount of information available out there we need to reduce things to a semi-manageable flow. But search engines and commercial web sites adjust results so they reflect what we've clicked on before. If you buy classical music, iTunes isn't going to show you acid rock on your front page (unless, perhaps, other people who buy Beethoven have an inexplicable preference for it). If you frequent news articles that have a liberal bias, you're never going to even see most articles that discuss things from a conservative point of view.
It's convenient to narrow the world somehow, but on the web we're not always even aware that it's happening. We may end up living in a universe of our preferences, rather than living in the real world.
Filtering on the basis of our likes means we lose access to diverse opinions, which erodes our motivation to reconcile different perspectives or even engage them. (How many "Can you believe those [right wing/left wing] morons said this" posts have you seen lately on Facebook?)
We lose an understanding of where the other side is coming from. In essence, we surround ourselves with the virtual equivalent of yes-men.
We also lose exposure to the randomness of life, to serendipity. That dramatically decreases the amount of discrepancies our brains have to grapple with -- and learning is a matter of trying to fill in gaps between what we understand and what we don't. It has an impact on creativity. Why? Because while search engines answer the questions we have, they don't necessarily push us to think of the questions we don't know to ask.
I'm not saying (and the book isn't, either) that the web is bad, or that it's wrong to have easy access to answers. We do need to be aware of how that kind of focus changes us, though.
Definitely a book worth reading.