Hi - we'd like to know the popularity of the voice chat feature in
instant messaging clients like Yahoo! Messenger and others. 
Preferably worldwide figures.

It seems that the four primary instant messaging services(ICQ, AIM,
MSN, and Yahoo!) have different versions of 'voice chat', including
PC-to-PC and PC-to-Phone, as well as PC-to-PC with video. 
Additionally, many voice chats connect directly from one user to
another without a specific server between them.  This, combined with
the newness of the field, makes it practically impossible to track the
use of the specific feature.
Yahoo! introduced voice chat in it's version of Yahoo! Messenger on or
about May 7, 2001.  I haven't been able to find specific introduction
dates for the rest.  I will keep looking.