Few additional thoughts:
The language used on Internet is determined by technology available for communication. The keyboard has 108 keys and 23 functions. That limits communication.
On the Internet, people not just talk, or publish to online blogs or chat. They are multitasking. So, when doing something else, just to respond to a post or an instant message people may find it easier to express them in ‘alternative’ or ‘slang’ form of English.
Typing is cumbersome process, especially when you are chatting or messaging to many people at the same time. Use of ‘slang’ does seem logical then. To keep communication flowing smoothly and linearly, one has to type faster and express emotions and feelings efficiently, especially when there is a ‘lag’. Hence the use of fast pace language seems to be desirable.
In commercial use of instant messaging, it’s a fast paced world, just to tell anyone that some meeting is happening at some other place or someone is coming for a presentation right now would take a long while to type in a formal letter format. But, with slang, ‘ur’ saved.
Blogs and Usenets groups might be slightly different. As, people could format their reply offline and then post.