While the primary objective was to raise awareness, I’m not sure how switching off the lights would do that explicitly. However, the main electricity operator said there was a drop of some 800MW, or 1% of France’s total consumption. 800MW is a decent amount of energy – a single modern nuclear power plant generates about that much.
The question from me would be: why don’t we do it more often, and for longer? There’s entirely too many lights on at night that needn’t be.