If RIAA goes to a college or ISP and asks for a list of people who use p2p programs the school or ISP is obligated by law to provide.

Once RIAA began to file charges against people using p2p, file transfers dropped by 15%.

The following are just my extrapolations based on what I have read.

I do not think that that RIAA will go after any more people then is necessary. They want to make a point, not start a war against illegal p2p users. I think that the 1000 subpoenas that have been issued have been planed to cause the most turmoil among the public in the news and among, the law makers, it would not be worthwhile for them to try and prosecute everyone. So they choose a handful of people that whose fall will produce the desired effect. That effect is not to stop illegal p2p one there own, but to get the government to step in whether it be local or federal. Why would RIAA try to fight this battle with there money when they can convince the rest of the population it is wrong and get the government to use its money?
I don't measure a man's success by how high he climbs but how high he bounces when he hits bottom.
--General George S. Patton