Skype is a free application for instant messaging and telecommunications. Its popularity is partly because it's supported by several platforms besides Windows and Mac. Smart phones, Kindles, smart watches and many game consoles have Skype compatibility. In addition, the software is versatile and bridges devices and operating systems which allows communication between them to be almost seamless.
Skype provides the standard features associated with most of the current messaging services like emoticons, file sharing, group chats and voice calling options. Video conferencing and real-time screen sharing for up to ten people is where it shines the brightest, though. Skype also offers VoIP to landlines in several countries as a paid service. Of course internet speed plays a factor in the quality of the user experience, but that applies to all video conferencing and VoIP software.
Unofficial desktop messenger for Facebook