Many people seem to assume that any internet-based poll is, inherently, unreliable as compared to other polling methods. However, Nate Silver has taken a look at how a variety of polls fared in the 2012 Presidential election and found that many of the internet polls did quite well, outperforming other methods:

...some of the most accurate firms were those that conducted their polls online.

The final poll conducted by Google Consumer Surveys had Mr. Obama ahead in the national popular vote by 2.3 percentage points – very close to his actual margin, which was 2.6 percentage points based on ballots counted through Saturday morning.

Ipsos, which conducted online polls for Reuters, came close to the actual results in most places that it surveyed, as did the Canadian online polling firm Angus Reid. Another online polling firm, YouGov, got reasonably good results.

This isn't to say (of course) that online polling is always accurate. It still very much depends on methodology (some online polls didn't do very well at all). But, it should put to rest the idea that online polling is inherently flawed or inaccurate.