Does imessage use data when you're using Wi-Fi?

I have an iPhone 5 and I'm texting a person in the United States (I'm in Canada) using imessage. I also have Wi-Fi. Does imessage use data when you have a Wi-Fi connection?

No it doesn't. Only when you are not using wifu and your 3G is on.

Nothing uses data when Wi-Fi is on

No. Nothing does.

No but really messaging doesn't matter, it uses very little data regardless.