That is definitely a very valid point. It is difficult to get the information about a study out to the public and potential study participants. Every day you can see adverts in trains, buses or newspapers, because those are the places where you can easily reach a lot of people. On the other hand, it might create some kind of bias for your study. I don't know of any better way to get people to enrol in a study though. So bearing the potential for a bias in mind, one should be able to proceed with the study accordingly.
