I came across a table about the most frequently used words in spoken and written genres in James Pennebaker's book on "The Secret Life of Pronouns", so I did a quick analysis on my cca. 300.000 words workplace Instant Message corpus. The search was nothing complicated, I defined words as string of characters divided by a space.
The beginning of the list is very similar to that of Pennebaker's (page 26), but some of the results are rather interesting.
There are no abbreviations in the top list, for instance. Check out you and u - both quite frequent, but the "traditional" spelling is far ahead its "easier to type, time/effort-saving" little brother.
Or have a look at the verbs. If I don't consider the auxiliaries, there is only one verb on the top list: call . People Instant Messaging at work don't write, don't type. They call. Or would rather call. Or ARE in a call.
Finally, the modest just at rank 30. Of course just has several meanings, but knowing my data I can tell that it is mostly used as an adverb in a minimiser function. An evidence for the fact that in workplace interactions people really have to make an effort to find the fine balance between getting things done and not offending colleagues. Just is a very useful politeness device to achieve this. Just think about it. (sorry for the pun:)
I'm sure that the corpus holds more interesting facts. I'll keep you posted.