For phones older than the android touchscreen ones but which have their color screen and can run java apps, there are 2 apps that let you chat using your gmail, yahoo or facebook account.
Ebuddy and Nimbuzz.
It's same kind of live chat as in whatsapp.
So, you can be online and communicate with ppl for free on a cheap 2G internet pack, without having to get an expensive android phone... the simpler java models are enough!
On my phone (sony ericsson k800, 6 yrs old, no touchscreen), ebuddy was able to get me online on gmail/gtalk; while nimbuzz got me online on facebook.
About whatsapp: well, it was aggressively promoted, made its own network instead of using existing ones, and purposefully stayed off of computers and older phones. Very limiting. It's their choice not to be backwards compatible. Too bad.