With Yahoo IM basically being completely dead due to their totally illogical redesign, and AIM really not being used either anymore. There's also ICQ, but when was the last time anyone used that? I remember people using that about 10-15 years ago.

The big three now are Skype, Telegram and Discord. Discord and Telegram being the new kids on the block, and Skype which was bought by Microsoft and merged with MSN (I remember using MSN Messenger alot before I started using YIM all the time). Now, as for Skype, its gotten worse for a lot of people (I never have troubles with it), but it has ads, and it requires some "advanced" editing to get rid of the ads. It also seems to be dying, being replaced with Telegram and Discord.

As for Telegram, its not bad, but I find its requirement of someone's phone number to add them as a contact kind of annoying. It is very basic, and doesn't even have the ability to use the /me function. To be honest, there's not a lot Telegram can do.

Now, with Discord. Its quite advanced. Each chat room is a server, and you can have different chat rooms within the server as well as voice channels. Its like a combination of IRC and Teamspeak. You have per server notification settings, and per server nickname settings. Your own nickname can be different in each server that you are in. Apparently Discord added a calling function (basically a private voice chat like Skype) recently. To be honest, I see Discord as the direct replacement for Skype.