Actually before AOL there were several crappy services, like Prodigy and Compuserve. No abbreviations there other than for 'grin', and the old standards like wtf, btw, fyi, and thx. I don't ever remember an 'np'.
And before that were the days of the BBS-- modems calling up single servers in some dude's basement. That's where your warez scene got started, and the whole replacing letters with similar numbers or symbols was just a natural extension of ASCII art.
We also never used smilies or frownies or any real abbreviations for short already short words, but we did use for the smiley there too. stfu and ggr and esa were also very popular, no surprise there. But no 'np'! I'm pretty sure about that because who says 'your welcome' when you're trying to be L33TX0RS anyway? :)