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-Varieties
Clipping (without apostrophe) - goin, til
Initialism - IYKWIM (if you know what I mean), FWIW (for what it’s worth)
Acronym - LOL (laughing out loud) [acronyms can be said as words, initialisms can’t - we say the letter names, like BBC]
Phonetic spelling - cos, fone
Letter and number homophones used for full & part words - u, 4, 2moro
Deletion of vowels - wd, yr
Smileys & emoticons (often downloaded now) - :-)
Explicit expression of feelings/attitudes- <evil grin>, *ducks*
Hashtags for ironic commentary on tweets e.g. #profoundtweet as well as used for topic marking