A recent Nature review of a PLoS ONE article resulting from research at Los Alamos's MESUR Project on citation and usage data analysis:
Butler (2009) Web usage data outline map of knowledge, Nature, 438, 135.
Bollen et all (2009) Clickstream Data Yields High-Resolution Maps of Science, PLoS ONE 4(3), e4803
MESUR Project Documentation
Pretty plots suggesting that because practitioners read widely but do not publish frequently, citations in the literature under-represent the reach of humanities and social science journals,etc titles.