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These are relatively new varieties and to a degree, are not yet fixed and standardised (or may not ever become so!). We are talking here about the language used in electronic forms like text messaging, email, chat/msn and tweets.
There are some differences between these forms, however, mainly due to the way people use them and the social groups users fall into. For example, I use email socially and am not a user of instant message services. I am also older than you (yes, really!). Thus, my email use may be fairly informal and I do use acronyms/initialisms, but I do not use txt other than on a mobile.
The main influences to people using these linguistic forms are:
Space limitations (ie number of characters)
Purpose in communicating (social, formal etc)
Features of E-