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Definitely all the above. One minute I'm in a serious discussion with someone about highly technical issues and the next I'm writing posts to a newsgroup from the viewpoint of my cat.
Online you can be whoever you want to be and nobody cares!
On the other hand if you try and fake things too hard you will be found out and summarily beaten to death on line!
Online relationships that I've had in some ways mirror Real Life™ and what I've seen is people tend to act "more themselves" on line than they'd like to admit.
What I mean by that is if your are an anal associative personality (fancy words for a much shorter reference to a dirtier part of the human anatomy) you tend to act like on online and in some cases folks like that are more so because of the anonymous nature of being online.
We've all met trolls online I'm sure.
On the other hand folks that are more mature and reasonable tend to reflect that nature online.
What I find sad in a way is there are folks that I've met online over the last bunch of years that I only know as online personalities and due to geographic limitations I'll probably never meet them in person. I guess that is better than not knowing them at all.