[pintxo]

Not sure the efficacy of mass surveillance can be determined within timespans of less than several decades. As there seem to be two major effects at play, one is the direct usage of information learned from mass surveillance which can be used by the surveyors. I can only assume this is the "works pretty well" you refer to. On the other hand, there are the (indirect) effects of mass surveillance on the surveyed population. Maybe the best example here is the GDR, which had a massive surveillance operations run by the Stasi (Ministerium für Staatssicherheit). They might have well delayed the ultimate dissolution of the GDR, but ultimately they could not stop it.