This bit both somewhat limits the impact and makes Greenwald et. al.'s claims that most everything is being Hoovered up a lot more credible:

"The XKeyscore system is continuously collecting so much internet data that it can be stored only for short periods of time. Content remains on the system for only three to five days, while metadata is stored for 30 days. One document explains: "At some sites, the amount of data we receive per day (20+ terabytes) can only be stored for as little as 24 hours.""

Of course, as the article goes on to detail, anything that's found to be of interest in that window can be saved permanently, and NSA analysis do that a lot.