Is Harvard referencing style used mostly in Europe? I ask because it seems to be less well known in USA than I had expected. If so, which is the most commonly used style in US academic institutions?
The distribution is not so much geographic as field-dependent. For instance, most of the papers I see that use the Harvard style come from the areas of geology, hydrology, and geochemistry, while most of the remainder use various numbered citation styles (RevTeX, AIP, ACS, etc.). This is true regardless of the geographic location of the authors, because the decision regarding citation style is generally made by the publisher, not the author.