Essentially, when people decide to participate in a survey, they select themselves. Just about all surveys suffer from self-selection problems. That is, when a potential respondent hangs up on a telephone based surveyor, self-selection has occurred. Likewise, when a potential respondent does not send back a direct mail survey, self-selection has occurred. The other issue is sampling.
Random selection is intended to ensure equal representation amongst populations.
To accomplish this, steps need to be taken to get respondents in a random manner, e.g. drawing numbers out of a hat. Our survey uses non-probabilistic sampling, which does not use randomization techniques to get respondents. This reduces the ability of the gathered data to generalize to the entire user population. We tend to agree, though have taken steps to further explore this issue. Specifically, we designed the Third Survey to enable us to determine how the respondents found out about the survey.