Does anyone know where I can get daily usage data for either the gnutella network or kazaa (# of users / volume of data) over the past few years? I'm trying to do some research on the real impact of the RIAA lawsuits, so the more detailed the data, the better. Thanks!

Unfortunately I don't think you will find any such data. The only history graphs of the gnutella network are the LimeWire statistics at www.limewire.com which are completely unuseable because of the frequent updates of their crawler software.

The next best thing would be the GWebCache statistics from Jonathan Atkins, http://gwebcache2.jonatkins.com/?graph_byclient
but they only give you an impression of the last two months or so.

If you are very lucky you can get confident data directly from LimeWire or BearShare who should at least how many clicks their websites have and how often their software has been downloaded.