I think it would be easier if you listed the ones you are not sure about since there are hundreds, if not thousands.
I cannot understand why people use abbreviations on forums (apart from TNT/ISO/REC which are useful for thread subject headings where space is limited). Abbreviations come about when it is necessary to get your message across quickly, such as in a real-time situation as per a chat room or during online gaming. Forums and email should be written out in its entirety as there is no need for speed as there is in the situations I listed earlier.
And no, I don't believe that it saves time when writing out a post or email, speed is not a necessity when using communication mediums such as these.