If your going for free try ebuddy free or IM+lite. Most of these apps just pause in the background and the message is on their server so until you view it you won't use any data. Most IM messengers are quite small data users unless your sending files or pictures.
oh cool thanks for clarifying that up. i used ebuddy before, but i cant message people who are on their phones through IM fowarding, and its a must for me. i may give IM+ a try as long as it has fb contacts also. again thanks