Cluster sampling should have been used.
Work Step by Step
The best would have been if the USA was portioned into different regions. Then, using the different regions, two people could have been randomly selected from different regions, and then it could have been seen if the person from the first region could contact the person from the second region. Finally, if the person from the first region could contact the other, he would have eliminated the problem of certain regions being more connected than others.