Can I just say thank you to Otilia for leaving such a detailed comment on my blog which I'm now going to respond to:
"Probably because Yahoo Messenger comes with much more scripts to be ran and accepted by one's antivirus & firewall and most people are having troubles in using Y at work because of that. MSN is, on the other hand, more work-friendly.
Additionally, Yahoo's people search is really nothing compared to the powerful search ICQ has, for example. MSN is no better in terms of people search, but probably people stick to it outside of work hours too just because they've got already lots of contacts on MSN.
I personally use MSN from work, and ICQ from home... Yahoo has no extra selling points to me, so to speak. The Yahoo music service would have been one, but the paid subscriptions are only available to US residents and that's not the case with me. Voila, then, why I stay on MSN and ICQ."
I agree with all your opinions above. It is likely that people are going to use and stick with the instant messaging system they can use at work, because that's where their contacts will be. ICQ I've always thought of as a minority IM application, I do have an ICQ number but have never really used it as I just don't have that many contacts on there. I also agree with your poke at Yahoo for making their paid music service US only. MSN have done something similar with MSN Radio Plus, something which has bugged me for a long time
However, the main point being is that people go where their contacts are and for some reason MSN has managed to obtain a very large user base which means even more people are going to use it.