Asset= Something that earns you money. Some websites classify for this definition then. As for how to value them- A multiple of monthly earnings.
They might be assets, they might be liabilities. You can't just slap a label on them without understanding what the words mean.
I voted no only because the majority of websiite's ad revenue isn't greater than the cost to run them. If cost is less than profits each month then it can be a very powerful asset