This is the question posed by two researchers in their working paper:
Did Online Access to Journals Change the Economics Literature? (SSRN)
Mark McCabe, adjunct professor at the University of Michigan’s School of Information, and Christopher Snyder, economics professor at Dartmouth College question the often reported 'citation advantage' to publishing in online open-access journals.
This Steve Kolowich article explains more about the study and its potential impact for the proponents of open-access publishing.