My impression is:

1. Dutch-style utility riders using a bike to get around town for errands -> Rare helmets

2. Lycra-clad competitive cyclists with drop handlebars -> Widespread helmets

3. City cycle hire scheme users -> Rare helmets

4. Parents cycling with young children -> Widespread helmets

5. Poor people commuting on beat-up mountain bikes -> Rare helmets

6. Organised cycling events with mandatory helmets -> Widespread helmets

7. Teenagers riding with their friends -> Rare helmets

So the popularity of helmets depends on your locale (does your area have utility riders?) - and whether you measure per-mile or per-bike-owner :)

But fair, I don't have numbers.