Instant messenging can be a great way to open up a dialogue. Some people however, treat it like their personal PA system.
Periodically they take off their away message (or sign in) and send out a few brief messages like these:
“Mark, if you want to hang out tonight call me.”
“Send me your status report ASAP.”
Then they are away again. If you are this person think twice, and maybe get a Twitter account.