Bruce Schneier describes data mining as How To Not Catch Terrorists:
Used properly, data mining is a great tool. As a result of data mining, AT&T reduces the costs of cell phone fraud, Amazon.com shows me books I might want to buy, and Google shows me advertising I’m more likely to be interested in. But it only works when there’s (1) a reasonable percentage of attacks per year, (2) a well-defined profile to search for, and (3) and a low cost of false alarms.
None of those criteria are true for terrorism.