Hi there.
As a former Chinese tutor (taught a Vietnamese) and a Singaporean Chinese with a good Chinese accent (lots of people asked if I'm from China

), I'd say it's near-impossible (possible but bloody difficult) to actually learn how to speak Mandarin over MSN!
Yes you can teach the hanyupinyin and the numerical connotations but it doesn't bring you very far in terms of proper pronunciation. Unless you have webcam and microphone...that'll be totally different.
But purely by IM-ing on MSN...trust me, the learning progress is zilch.