I would say facebook

Windows Live was most popular in my country Saudi Arabia

Right now Wathsapp is most popular

Hi, here in Syria sh messenger is usually used on mobile, I like it alot , I don't know if it's famous any where else , also so many people use fb chat most of the time ..

VKontakte, Skype, QIP or ICQ, Mail.ru

it is in decreasing popularity )) 

Facebook, Whatsapp & Skype in Germany :)