I think i have read somewhere that IMs are more formal than a phone call, but not as formal as an email, which isnt as formal as a snailmail letter
(recap [non formal] phone | IM | email | letter [formal] )
also as has been stated many times, its free, and can help you get in touch with people you normally wouldnt.
and on the note of formal, some people abuse ims making them into another language that is incomprehensable to some. and if i were to stick irc on that scale i'd rate it under phone