Skype is the most used instant messaging system right now. One of the very popular on demand apps and is used by many all over the world.
I got this picture from this link: http://www.macrumors.com/2013/03/07/skype-updates-ios-app-with-new-calling-experience-and-bug-fixes/
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Tencent qq is mostly used in China and it is used by millions of people over there.
I got this picture from this link: http://blog.gmeochina.com/?p=262
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Facebook Messenger is also very popular now because a lot of people in this world use Facebook today so it`s a good thing they have an Facebook Messenger/Chat for friends to communicate.
I got this picture from this link:
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Also, there is iMessage by iPhone. This app is specifically used by iPhone lovers and not only you can iMessage for free with your fellow iPhone users, you can also FaceTime, a Face-to-Face conversation with friends.
I got this picture from this link: http://appadvice.com/appnn/2013/03/apple-faces-yet-another-security-issue-in-the-form-of-imessage-denial-of-service-attacks