You do need to be careful about those things. I doubt the adcom has time to really look into you, but with just a email or a username they can generally find their way to your facebook, your youtube account (all the comments made by you), etc, etc. There are all sorts of cookie crumb trails to follow if a person has enough time on their hands.
The most shocking thing I heard first hand was what my roommate did. He posted on one of the 'school specific' boards here at SDN and said wrote something along the lines of "Well I'm meeting with the dean of admissions on Wednesday morning" Yea, turns out the dean occasionally browsed the thread (since it was school specific discussion about his school) and then the dean mentioned it to my roommate during their meeting. 😱
So yea, everything my friend posted on SDN over the years was easily linked to him and his application. He was a good guy though and he got in regardless. But still, it makes you think.