From in the privative side, Facebook has all the power because all the people are behind Whatsapp and also is the owner of Messager but Slack wins on the business world, Fortunately Skype is dead.
In the Free/Libre side, we have many options and is hard to understand for a new user what is better, we have Signal/Kontalk/XMPP/Tox/Matrix/Sip/IRC etc.
and in the middle Telegram with a private server and free/libre client.
Kontalk will not be useful until be compatible with other XMMP clients/servers, I still don't feel comfortable with Matrix, Tox doesn't have a usable client on Fdroid, Matrix will replace XMPP? For now, XMPP/IRC will continue to be my best options, I'll give another chance to Matrix but Riot.im is really annoying with the notifications.