While I agree that integrated and invisible is the way to go, the popularity of iMessage will be limited by how many people actually have iPhones. While I know of a lot of people with them, it's certainly not the majority of people I know.

Whereas with Facebook, just about everyone has an account.

The killer feature really is group messaging on-the-go with all of your friends, regardless of their phone. As it stands right now, I have a feeling Facebook are the only ones really going for this.