girl: What's wrong with that girl in the corner?
guy: Hahaha! She just got salmoned.
It's similar to a three way call using AIM. Imagine someone mutes his own phone and then calls you. Then he calls someone else. Now you're talking to a stranger. Your phone rang and so did theirs. You think he called you and vice versa. Neither of you realize there's a third person instigating all this. This is basically the same thing, but with AIM.
Me: So you realize we're being salmoned right. And your s/n is showing up as AllSeeingSalmon.
AllSeeingSalmon: wtf are you talking about?!
AllSeeingSalmon: no this is Jim
AllSeeingSalmon: I already told you that!
Troy: Because I keep getting salmoned. Let's play Halo instead!