http://www.pewresearch.org/2015/09/22/coverage-error-in-internet-surveys/?utm_source=Pew+Research+Center&utm_campaign=e2ad048820-Methods_Newsletter_for_June6_24_2015&utm_medium=email&utm_term=0_3e953b9b70-e2ad048820-400071421

"…and surveys that include only those who use the internet (and are willing to take surveys online) run the risk of producing biased results. How large is that risk?"