Yahoo IM
Usage of the different IM platform varies a lot by country. The UK is really dominated by MSN, while their penetration in the US is much lower and more users still use AIM and Yahoo. I would guess that Europe uses more Yahoo than the UK, with odd things like GaduGadu still dominating in Poland. No idea about non-western countries, although good character set support may well be an issue that drives users to specific platforms.
The other issue is which clients get included with / are easily available for which mobile phone. I know that WinMo comes with an MSN client, but I don't remember any client supplied with Android or iPhone. Many of the more effective clients like Meebo, which handles all the protocols in one screen probably register as HTTP traffic anyway.