A lot of these abbreviations come from chat rooms where you have to type as fast as you can just to stay in one place. USns is a play on a substandard English version of the first person plural pronoun. The word us is the first person plural accusative and dative pronoun. Using it as the first person plural nominative is a sort of self depricating play on the notion that we USA people are ignorant yokels. We used to say merkin in a similar manner (short for American) but some of the non-USns couldn't shake the notion of the female pubic wig and their giggling was a little annoying, so we quit that.

You might want to PM Emanuela about her difficulties in keeping up with us, difficulties that she overcame to a large degree by coming to last summer's symposium we had in central Michigan.