The five sites i think that has the Do(s) :
1. Facebook.com
2. Gmail.com
3. Youtube.com
4. Twitter
5. Google

These site are well recognized firstly because of its simplicity meaning to say that the content is very simple n easy therefore giving the best user experience . Mostly these sites are popular among the youths because they are able to adapted to this sites and these sites seem to be credible. For me these sites are good for social experience and sites specially for young minds.