Is it okay to break up with my long distance boyfriend on instant messaging?

I would say don't do it. It's not a very nice thing to do, and it also could be taken the wrong way (he might think someone else got onto your account, playing a joke, etc). At least give him the courtesy of a phone call or skype call so you can explain things to him. It would be really awful to be on the other end of things and only receive several lines of text that end your relationship.