In particular, this part: > It seems that it could be made trivial to narrow down lists of suspects by crunching large data sets that contain stuff like SO que

In particular, this part: > It seems that it could be made trivial to narrow down lists of suspects by crunching large data sets that contain stuff like SO questions, AC posts on Slashdot, or responses on HN. Is what XKEYSCORE does (or at least, it's the interface you use to query the above data sets which are collected as part of PRISM and the other programs).