The researchers Brooks cites find that we actually encounter viewpoints we don't share on the web. An example that occurs to me is on the conservative site Lucianne.com it is not uncommon to find someone posting link to a Frank Rich, Joe Klein or Maureen Dowd column.
Similarly, we sometimes find links to work by MSM liberals on RealClearPolitics. COTTonLINE rarely links to explicitly liberal material but often links to scientific or international material with no political tilt whatsoever.
Brooks' conclusion: we are getting more polarized but the Internet is not, as often claimed, at fault. He doesn't answer the next question: what is at fault?