In conjunction with ISPs, the (US) government or a large private company like Google or Microsoft could aggregate posts based on IP address. In fact, according to a report by Frontline ("Spying On The Homefront"), Homeland Security started implementing such a system for monitoring all phone and Internet communications after the passage of the Patriot Act. I was going to say that it would probably be political suicide for a private company to participate but I just remembered Homeland Security actually contracted a private company to do this work for them. I can't remember the name of the company but you can watch the whole show online:
Using writing style seems a little far fetched given the limited scope of variations in language relative to the large number of people using language. Otherwise it would not have taken so long for effective speech recognition software to appear - even the current incarnations are more fragile than one would expect relative to how long the best minds have been looking at the problem and relative to the high economic value of an effective solution.
http://www.pbs.org/wgbh/pages/frontline/homefront/
Using writing style seems a little far fetched given the limited scope of variations in language relative to the large number of people using language. Otherwise it would not have taken so long for effective speech recognition software to appear - even the current incarnations are more fragile than one would expect relative to how long the best minds have been looking at the problem and relative to the high economic value of an effective solution.