The law says it applies if more than 30% of the content is adult content. So won't apply to reddit or Twitter at least but I could see this still leaving plenty of gray area for other sites.
I wonder if these adult sites could host a ton of non-adult content in order to get below the 30% mark. There used to be a similar law in (I think it was) NYC stores. To combat this, adult bookstores would stock tons of non-porn titles which weren't even really for sale.