Mass Surveillance and the Spies We Trust

As we know from news reports, many countries are engaged in mass surveillance of various forms. Private companies are also engaging in various and sundry forms of large-scale data collection of their own, some of which can be fed back into governments' hands. What, though, is "mass surveillance"? Is it ever justifiable? How necessary is it? We all know that there really are Bad People out there, though of course there are many different opinions on just who they are. Does mass surveillance work? If it does but is immoral, are there alternatives that are more palatable but equally effective? How much diminution in effectiveness is acceptable?