you could also use softwares to log into messenger, which is always a better bet than using wap messengers. And they dont use much of a trafic either.
For low data usage, you could use Mig33, supports Yahoo, MSN and AIM. And it is free.
But you should be having a JAVA enabled phone.
PS- BTW, if you decide to use Mig33, eventhough you can download and register for the service free of cost, if some other Mig33 member invites you to the service, he could get 1 Aus. Doller credit for that, which can be utilised to send cheap sms and make cheap calls. You shall get your 1$ credit on registering eitherways. So why not do so through an invitation
I am a Mig user, and need to make regular calls to USA, hence, it helps me to invite people. If you dont mind, you coud PM me your number and i could invite you to Mig?