Lovely Bicycle asked how the private cyclist population in Paris (not on Vélibs) compares to the cyclist population in Boston.
I would estimate that it is a slightly larger percentage of a certainly larger population than Boston's. I offer as corroborating evidence, these photos of bike parking near the Hotel de Ville for a free summer evening concert by some unknown band. In Boston we only see this kind of bike density for bicycle-related events like the Redbones party. Here one would have trouble finding a spot to park, that's never been a problem for me in Boston. It's worth noting that the nearby Velib stations were also full.
Click to expand the photos; a prize of some kind for the first person who can count all the bikes!