They don't sniff all the packets to find anything useful. Snowden showed that they intercept certain packets flowing between particular sources and destinations that might contain useful information. In Snowden's leaks, one such application was email inbox backup transfers for big Internet companies that did not (at that time) encrypt their WAN traffic, from which they mined the sender and recipient to build a social graph, a program that the leaks said had already been shut down.

Mining public forums like this one is likely fair game and seems not to be a secret, but it doesn't require anything in Room 641A.