Microsoft AdCenter (the home of Microsoft Digital Advertising Solutions) has displayed a few interesting statistics on their website. They seem a bit off to me, after spending time with Microsoft I know of the figures then but not as such now… so they may be for the world or certain parts - I think they’re for worldwide…

MSN Hotmail
33,170,000 unique users

MSN.com
46,268,000 unique users

Windows Live Messenger
28,055,000 unique users

Microsoft Office Live Online
18,113,000 unique users

Source: Neilsen NetRatings December 2006