I'm not sure I would agree with "routinely" but yes certainly people make mistakes and wish they could take back some things they post. I encourage you to thumb thru Trayvon's tweets and reasses what is or is not out of character for him.
No, because I maintain 90 percent of people who post on the internet use their internet personality and not their real personality.
Internet postings are not really meant to be taken seriously, so you really cannot take it as reflecting reality.