I prefer MSN for study, for precisely the reason that Harregarre mentions, also, on MSN you can activate your microphone without requiring a two way conversation to start. Skype requires that both parties speak. Sometimes it is good to practice by reading text aloud as another person types it. Though you can do this with Skype, language partners that don't have a microphone or cannot use one at the time would still be able to hear you on MSN easily, with skype it is a little harder. I can't say anything for Yahoo.