From what I understand, in China (at the very least, to my knowledge) the phonetic sound for 88 in Chinese is similar to saying "bye bye" in English, which a lot of people in China say. So when they say "bye bye" in a text message they say "88". That's all I can gather from it.
Another neat thing: I think it's in Thailand or Vietnam, I cannot remember, the sound for laughing sounds like "5" in that language. So on facebook or in texts or IMs they'll say things like "5555555555555555" to mean laughing!
Oh man, I love Minecraft more than I probably should. So so SO fun!