I didn't even know it existed until I got a message saying that my story was not following the rules... so I read it and it was a really minor thing, but I fixes it...
To be honest I think many people are not familiar with the rules. When I first came on here I was using chatspeak a little bit, mainly because I had NO idea whatsoever that you weren't allowed to use it. They should make the rules more easily accesible.
http://www.mibba.com/forums/topic/3737/ These are the rules for stories... they also go for stuff like journals, or most of them do.
Just to warn you, they are strict about using proper caps and punctuation. People don't correct it to be mean, they just don't want to see you get kicked off Mibba!
I'm not sure but it's definitely worthwhile to be grammatically correct all the time. People always pick up on it otherwise. Welcome to Mibba anyway :) If you have any problems, comment on my profile.