Telegram is fast and light. It allows you to change themes and is really feature rich. It uses up to 100 MiB on x64 system.
Now Viber. It is buggy and clunky. You have to make strange hacks to work it nice on Linux desktop. It uses 300 MiB from the start. They are working to make it better, but it's too late for me.
This Mastodon instance is for people interested in technology. Discussions aren't limited to technology, because tech folks shouldn't be limited to technology either!