As of march 2018 the Tor Messenger has been discontinued - Tor Project as well as us don't recommend using it anymore!
An Alternative, which suits high security standards could be Riot Messenger based on the Matrix protocol. It's available for all big operating systems.
Further infos on this can be found here: https://about.riot.im/
Also Signal is a messenger that's often recommended by security experts.
Torproject.org has recently released the Beta version of the Tor Messenger. You can use the Tor Messenger in combination with our Jabber/XMPP service – anonymously and without fear of unwarranted surveillance.