Here’s an interesting study of the .com domain names. It appears that only about 1/3 of the registered domains are in use by legitimate websites. The rest are either spam, email-only, empty, broken, etc.
Only about 100,000 domains were crawled to provide a representative sample. But to me, the numbers look quite realistic. If only, I would push the porn and gambling sites into the “in use” category, rather than have them separately.