When you IM somebody you've never IMed before, you damn well better say who the hell you are first thing, before I have an aneurysm trying to force you to by sheer will. GAAAH.
If you don't say who you are and I can't immediately tell (for example, by looking it up), I'm going to assume you're some spammer, grifter, or bot trying to tag my handle as live, and act accordingly. Just so you know.