I hadn't come across this practice until I visited Paris last year and came across Pont de l'Archevêché at the back of Notre-Dame Cathedral which was covered in these padlocks (on the cathedral side at least), some with ribbons, some with little love messages, initials, dates, etc. I think the authorities must clear them out every so often as the Google street view shows nothing! From memory, I think there were some more padlocks on one of the other bridges towards the other end of the island, or perhaps crossing over to the Louvre, too.