IRC4Life!!
of course, I'm logged in to talk to people I play a '90s text based strat game with. I'm totally with the times :look
(We use slack as well for private chat, but general is way better on irc) ;)
I would honestly take WhatsApp off the list Elania. It requires you to give out your phone # to be in the "chat" and would be seen by all. And I think that a number of people would be opposed to that? (But I could be wrong....)
I use androirc on my phone, colloquay on my ipad, mirc on my computer. There are web apps that allow you to use mobile as well. *Shrug*
Plus I'm on IRC all the time already for other things, so to me it's just one more server open :)
My issue with WhatsApp is that you have to give your phone #...