The top 5 most popular websites used in the United States of America are:

·      Facebook which has approximately 750,000,00 users worldwide.

·      Twitter which has approximately 250,000,000 users worldwide.

·      LinkedIn which has approximately 110,000,000 users worldwide

·      MySpace which has approx 70,500,000 users

·      Google Plus + which has 65,000,000 users.

http://searchengineland.com/a-portrait-of-who-uses-social-networks-in-the-u-s-and-how-social-media-affects-our-lives-81653