Enough options on N900 to have a 'chat'. I am using email, sms texting and Telegram. The first two probably the most future proof and more importantly everyone is able to use those without any effort.

I've noticed not many people like to install an app just to chat with the only user in their group of peers that isn't able to use whatsapp. Even installing an easy to install and pretty widespread app like Telegram is usually too much trouble. Still I regularly use it with a couple of friends/family.

Haven' t investigated but I am sure there are other options as well on the platform. Pidgin has some plugins that might work (eg AIM, ICQ, MSN, XMMP)
Probably more options as well if you use Easy Debian. But as mentioned before, you need to convince your peers to install the same program.