The rules for texting, where are they written? who wrote them? That is something I lack knowledge of.
Common sense. Unless you have a Blackberry you can only put about 140 characters in a message(the Tweet). So you have to CONDENSE what you want to say. In english we would use 1,2,4,8, C, U, N, Y for example to shorten words but never lose the context of what we are saying. It makes no sense to use alternate words instead of the appropriate word if it has the same amount of letters(or more) as the initial word you were going to use. This what makes me believe that this woman is writing phonetically as she speaks.