Shanghai blogger Bingfeng has a post about Chinese instant messaging slang. Here are some of the terms from his list, reproduced in a slightly edited form:
pp = 片片 (pian pian) = picture
mm = 美眉 (meimei) = girl
ppmm = 漂亮美眉 (piaoliang mei mei) = pretty girlfb = 腐败 (fu bai) = corruption = dining out
bt = 变态(bian tai) = perverted
opa = 好的 (hao de) = OK
ding = 顶(ding) = support, agree
3x = 谢谢 (xiexie) = thanks
88 = 再见 (zaijian) = bye-bye
286 = 落伍 = out of fashion