"[l]ast year, fewer than 300 phone numbers were checked against the database of millions of U.S. phone records gathered daily by the NSA"
>>For future reference I might believe somewhere in the range of 15,000 to 50 million
You think that there are at least 15,000 terrorists/foreign targets of interest that would have any reason to call a US number?
Let's take the Boston Bombers and make a low balling guess that each one of them has called 20 unique numbers in the last 6 month, each of which have in turn again called 20 unique numbers and we are already at 800 numbers under surveillance.
For instance we know some details about the MINARET project which was not the full extent of their spying that:
"Operating between 1967 and 1973, over 5,925 foreigners and 1,690 organizations and US citizens were included on the Project MINARET watch lists. NSA Director, Lew Allen, testified before the Senate Intelligence Committee in 1975 that the NSA had issued over 3,900 reports on the watch-listed Americans." - http://en.wikipedia.org/wiki/Project_MINARET
One imagines that with the growth of technology and the growth of the NSAs budget from that time they are able to distractedly increase those numbers.