Sorry, HKEx, your numbers are wrong and outdated. And is sounds like the smokescreen the ALA uses to cover its action in defiance of the US Supreme Court. SCOTUS says, unanimously, that it is "legitimate, even compelling" to keep minors from inappropriate material. The ALA, on the other hand, say that's "age" discrimination. Hmmm. Which to believe?
Fine, let's change the numbers. If web filtering is 99% effective and there are 20 million pornographic web sites out, then that means there are 200,000 pornographic web sites available to children.