I was talking to my ENTp friend today. I mentioned one time that I could have him come here and tell you guys why I can't be ENTp. He thought this was hilarious, but also said something I didn't expect: he acts more reserved online than IRL. (Though it should be mentioned that he has limited experience posting in forums.)
We figured the reason for this is that he is used to, and appreciates, the spontaneity of RL conversations. This is his natural environment for being witty and extroverted. I, on the other hand, like the time to plan out what I'm going to say online, and thus when online act more witty and extroverted than normal.
How does your personality (not) change online vs. IRL?
Since my own typing has been called into question based on my online behavior, I have to constantly remind myself that other posters may not be quite what they seem either.