Can anybody find figures about # of unique users worldwide using IM programs.
What is the marketshare of AIM,MSN, Yahoo etc?

Figures of 2003 

Although AOL's AIM and ICQ together make up the largest IM network,
MSN and Yahoo are making strides.
In March 2003, 
AIM had 31.9 million unique users 
ICQ had 28.3 million,
MSN Messenger reached 23.1 million unique users 
Yahoo Messenger reached 19 million.
 
http://news.com.com/FCC+lifts+AOL+messaging+limits/2100-1032_3-5065650.html

Also found an analysis of the Enterprise Instant messaging market for
2004-2008 however the prictag of this report is $2500,=

http://www.gii.co.jp/english/rd20357_instant_messaging_toc.html

Hey there,

This took a lot more work than I initally expected.  Here is what I
found:  The general consensus is that there are a combined 400 million
users of AOL, MSN, and Yahoo instant messengers.  It is easy to find
stats on how many users each company had several years ago - finding
current stats proved to much more challenging!

AOL apparantly does not release its usage statistics to the public. 
However, I did find that AIM is the currently the most popular instant
messenger taking up approximatly 50% of the market share.

ICQ has 135 million registered users, however a good percentage of
these are no longer active.  (www.icq.com)

MSN has 130 million users (according to Blake Irving, corporate vice
president of the MSN Communication Services and Member Platform Group
at Microsoft -  16/07/2004)

The only other relevant data that I was able to find was the top 5
applications according to a study done in January of this year:

AOL Instant Messenger - 20%
MSN Messenger - 19%
Yahoo Messenger - 12%

(http://www.electricnews.net/news.html?code=9386218) 

I am sorry that I was unable to secure more concrete data for you. 
However from my research I would say that AOL is the definite leader
followed by MSN, Yahoo and then ICQ.  I hope this helps and good luck
in your search!

Josh Fraser

Sources:
http://news.com.com/2100-1023-963699.html (October 29, 2002)
http://msnbc.msn.com/id/5439702/  (July 15, 2004)
http://www.theaustralian.news.com.au/common/story_page/0,5744,9908120%255E15306,00.html
http://www.geekzone.co.nz/content.asp?contentid=3047 (16/07/2004)

Interesting, but outdated:
http://www.bizjournals.com/sanjose/stories/2002/06/17/daily2.html
http://news.com.com/2100-1023-963699.html
http://www.usatoday.com/tech/techreviews/products/2002-08-19-webcam_x.htm