Your comments regarding contamination do not sound plausible to me. If you're infected, of course your mask will also be contaminated by SARS-CoV2, but you hope

Your comments regarding contamination do not sound plausible to me. If you're infected, of course your mask will also be contaminated by SARS-CoV2, but you hopefully will not distribute as much of it around you as you would without the mask. If you're not infected, the only way to contaminate the mask with SARS-CoV2 is to be exposed to infected people exhaling it. And in that case it's better for your mask to become contaminated than to inhale it unfiltered. As for the "announcement effect", I did start wearing a mask when the mandatory mask policy was announced, before it became actually active in the following week. And I wasn't the only one that did that, so some announcement effect is very plausible to me.