An interesting Economist post on research done to look at the citation impact of having more and more scholarly output available to researchers.What he discovered was that, for every additional year of back-issues of a journal available online, the average age of the articles cited from that journal fell by a month. He also found a fall, once a journal was online, in the number of papers in it that got any citations at all. Indeed, he predicts that for the average journal today, five extra years’ worth of online availability will cause a precipitous drop in the number of articles receiving one or more citations—from 600 to 200 a year. Rather than measuring the length of the tail, then, it seems that modern science is actually focusing on a tiny bit of it.
The original article (needs a Science subscription) is worth reading, as the Economist piece does over-simplify the conclusions. For example, the author also discusses other work that disagrees with the article under discussion - science in action :-)
Also worthwhile is this user comment from the perspective of a student doing research in the modern library