Live Messenger usage is high (most popular service) in some countries (Belgium , Netherlands , Brazil, Argentinia) but Skype is much more known outside the more narrow scope of Messenger.
Also know, most people I know still say "Add me on MSN" because it used to be called MSN Messenger instead of Live Messenger.
Right now if I am 'online' I need to start: Live Messenger, Skype, Facebook, GTalk, Communicator (Lync) and so on... Anything that will help do this in one program is good for me!
# Skype contacts in Messenger
# Decent Skype UI ?
# Unified contact list
# Video chat with people on skype
# Video chat from messenger to any mobile device having support for Skype
# Video chat in WP7 integrated into contact list and their 'online' status.
This is just a great vision of connected screens. Altough I feel like they have paid a bit to much.
Lets hope they don't remove crossplatform support and really support all devices.