A new study showed that people believe that email requests are just as effective asking in-person. The study, published in the March 2017 issue of the Journal of Experimental Social Psychology, told 45 participants that they would have to ask 10 strangers, either in person or via email, to complete a survey with no compensation. Participants in both groups thoughts that one or two people would complete the survey but they were largely incorrect. More than 70% of people approached in person were willing to complete the survey. While only 2% of those asked via email were willing to comply.
What about soliciting a friend of colleague? Face-to-face interaction is still the best method. When a friend or colleague asks you directly and asks for a favor in person, it means they are in serious need and respect you enough to pay you a visit.