What I choose in the list top 100 are Facebook and Skipe.
Facebook is one of the famous social networks in the world. It is different from others in that it is easy to find friends. If a person add another person as a friend, he can know who is the friend's friends.
I think it is best of best because it make people more social and it is so famous that everyone in the world know what it is and how to use it. English is a global language. It will be helpful to make foreigner friends to use communicate in English on Facebook. http://www.facebook.com/
Skipe is a instant messaging tool. when I want to contact my friends or when I want to play games with my friends with talking, I used to use it. It's free. I think Using it with foreigners for improving learners' English speaking ability is good. Also some people, including me, say the sound quality is better than other tools.
http://skype.daesung.com/main.asp