The Presidential Daily Brief (commonly referred to as the PDB) is a highly-classified document designed to tell the President each morning what is happening in the world, and what people are thinking, with a focus on intelligence and national security. So the ability to scrape Facebook (say) and tell the President that people in Lebanon are really angry about Syria would be useful information. If the data suggested that there were large numbers of people talking about taking up arms and joining one side or the other, that would be really important intelligence.
Keep in mind, that Facebook has 1 billion members. Keeping track of that many people on a minute-by-minute basis would be nearly impossible without tapping directly into Facebook's servers. But the intelligence coup of tapping into Facebook's servers goes well beyond profile posts. Facebook has email messaging, chat, voice and video calling. If the government were tapped into that, they could follow anything and anyone they wanted.
And per se, if the information they're gleaning is making its way into the PDB, clearly they they think they're finding some good intelligence. That's not to say that this is a program the rest of us should welcome.
Allahpundit of Hot Air explains why the government bothered to keep the program a secret:
In other words, without revealing which particular tech behemoths are participating in PRISM (and I use the term "participating" loosely), what national-security harm could have come from announcing that, yes, most Internet communications are also being data-mined and occasionally intercepted for intelligence purposes? One obvious possibility is that if the bad guys know that the entire Internet's being bugged, they'll get offline altogether. That's good news in the sense that it'll make communicating/planning harder, but it's bad news in that it makes them harder to find. The guy responsible for 9/11 did pretty well staying hidden for 10 years by communicating via courier only. If the CIA has a net over the entire ‘Net, then naturally they want jihadis to use it. Also, it seems only logical to me that if you're gathering this much data, you're not just using it to spot terrorists. The feds are surely using it for foreign espionage purposes too. Imagine what kind of foreign-policy puzzle pieces they're finding by harvesting e-mails, even from private accounts, that foreign diplomats are using to communicate with people back home. Now that PRISM's out of the bag, countermeasures will be developed.
Timothy B. Lee of Wonk Blog said President Obama's assertions Friday that Congress was fully aware of the program doesn't mean they were actually privy to all the details necessary for proper oversight:
Obama's comments make it sound like the programs are subject to rigorous and continuous oversight. But the simple fact that Congress is briefed and federal judges are involved doesn't mean either branch is actually able to serve as an effective check. The excessive secrecy surrounding these programs makes that unlikely.
Take Congress. When the government has briefed members of Congress on its surveillance activities, it has often been in meetings where "aides were barred and note-taking was prohibited."
It's impossible for Congress to provide effective oversight under those conditions. Members of Congress rely on staff to help them keep track of legislative details. They need independent experts to advise them on complex technical issues. And they need feedback from the constituents they ultimately represent. But the senators briefed on these programs couldn't speak about them. Sens. Ron Wyden (D-Ore.) and Mark Udall (D-Colo.) were reduced to spending years trying to hint at the existence of programs they weren't able to actually tell anyone about. Only now can anyone see what it is they were trying to tell us.