Representativeness of the sample
All sampling methodologies have some limitations preventing
them from being 100% representative of any given population.
Just as telephone studies have limitations due to the problem
of unlisted numbers and cellular phones, and mailout surveys
due to such things as restricted access buildings and bad
mailing addresses, internet surveys have limitations.
You can only claim to represent those who have access to
the Internet either at home, at work or from some other location.
Current levels (2006) of Internet users are around 68%
of households in North America.
This population is perfectly representative of populations
such as online users or any subpopulation among Internet Users.
It is often appropriate for populations perceived to be hi-tech
users or in many business-to-business studies, as these populations
will also very likely be Internet users.
It cannot and likely will never be able to claim to represent
the entire country's population, or all adults 35-50 years
of age, or any other population that may have a significant
proportion of non-Internet users. However similiar things
can be said of all methodologies.