Should We Trust Web-Based Studies? (This research says Yes.)

This comparative study of Web-based survey samples and paper-and-pencil surveys (by Samuel D. Gosling, Simine Vazire, Sanjay Srivastava and Oliver P. John and which appeared in American Psychologist) is now four years old, but its conclusions are probably still valid: Yes, Web-based self-report and self-selected samples can be trusted for surveys. From the study's summary:

"Internet data collection methods, with a focus on self-report questionnaires from self-selected samples, are evaluated and compared with traditional paper-and-pencil methods. Six preconceptions about Internet samples and data quality are evaluated by comparing a new large Internet sample (N
361,703) with a set of 510 published traditional samples. Internet samples are shown to be relatively diverse with respect to gender, socioeconomic status, geographic region, and age. Moreover, Internet findings generalize across presentation formats, are not adversely affected by nonserious or repeat responders, and are consistent with findings from traditional methods."

Get the whole study here.