Recruitment has changed over the years with a lot more taking place online. We use all methods of recruitment tailoring each project to the most appropriate method. Whichever route we take, it is always important to have personal contact with the respondent both to increase the chances of them turning up but also to help verify that they are genuine. On occasion we ask participants to bring along some form of ID.
Here is an article from the AQR In Brief magazine from November/December 2011 talking about the various methods of recruitment. Recruiters themselves also use a combination of methods with many embracing digital.