Never EVER fall for the 'give your phone #' gimmick. Just throw in a 'dummy' one, and jot your password down elsewhere so you don't forget it.
I've used AIM, Yahoo, Skype and MSN. MSN no longer exists....they merged their contact lists into Skype and basically shut their own servers down for good (they're busy trying to compete with Google with their Bing and Outlook services). Ever since Ebay spun Skype back off into its own entity, they've been on a tear lately....their user base has been exploding in growth.
ICQ is a name I've not heard in years. In the West, they merged with AIM years ago (ala Skype + MSN have done recently), but their standalone product is still fairly popular in Middle Eastern countries and the like. They're the oldest IM service around.
Speaking of Google, their Talk service is, I believe, also being shut down, if not done so already.
For all intents and purposes because a lot of people are cutting the cord and ditching laptops/desktops (for non-work related things like friends, roleplay and other things), a lot of people quit using IMs because they are too clunky on a smartphone...not to mention their services (SMS, IM) can all be done on the fly and in streamlined fashion on Skype. It's usually Skype or plain texting for quick contact, I've observed.
I don't know how helpful all this will be, but hopefully it is of some use.
PS: Odd how most forums still have MSN as a field. Should be 'Skype ID' or something. Their IM side of the service allows usernames, even though a lot of folks go by their cell # with the service.