It goes back hundreds of years! National intelligence agencies have consistently managed to get full intercepts. Way back in the day, they counterfeited wax seals and steamed open letters. In the 1800s, they got telegraph printouts. In the early 1900s, wires. In the 60s-70s, tapes. Then hard drives. It's just part of what governments do ;)
I think that the big difference today is that they can analyze all data that they can grab. Back in the Good Old Days, you had to devote considerable manpower to this analysis; if you wanted to read letters, you had to be specific because you simply didn't have the resources to look at everyone's mail. This is why most people (including me) have no problem with the idea of monitoring POTS lines; it ties up a considerable amount of resources, so the police are much more likely only to tap the phones they think will get results.
Now, we've gotten to the point where the challenges facing mass surveillance are political rather than physical. I think that this is a lot more dangerous than 1800s police looking at telegraph printouts line by line.
Yes, that's an excellent point. And capabilities for data analysis are improving rapidly. Google still has far better tools, but the NSA has the intercepts. And it will get the tools. Eventually, it will become the Eschaton ;)
