> what value I derive from buying gasoline at the petrol station. None. I derive no value from it, I just spend money there.

The value you derive is the ability to make your car move. If you derived no value from gas, why would you spend money on it?

And likewise, presumably the users are getting something they want in exchange for having ads blasted at them.

If they just wanted ads blasted at them, and nothing else, they'd be doing something else, like, say, watching cable TV.