Abbreviations go back quite some way (SPQR, anyone?), but the current crop, I think, have their origin in hams using Morse code: BCNU (be seeing you), CUL (see you later), etc. Some have stayed with hams, e.g. YL (young lady) and XYL (ex-young lady, often someone's wife).
That made its way into hackish usage and thence into IRC and to text messages everywhere.
(OK.. maybe Prince had a hand in it.)
The international nature of SL means that one can see similar shortening in other languages. The one I have seen to date: Spanish speakers typing "k" (which isn't used in Spanish save for borrowings from other languages and foreign proper names, or things like radio station call letters) in place of "qu". Do they type ADM (ay, dios mío) where in English we'd type OMG? IDK. :)
I'll keep my eyes open for other instances.