I'm not really sure. If you were to ask me if IM-ing was intrinsically bad/evil/wrong, I'd answer "no". However, I believe it could (possibly) reinforce some bad habits. There's some things people say online that they wouldn't say to someone's face AT ALL. (I've found that out by surfing sites like Youtube + GameTrailers and...personal experience.)
I don't think that it's bad in itself. But proceed with caution.