Google runs the largest advertising network in the world. Plenty of malware slips through the cracks every day, both downloadable apps/software/extensions as well as ads that lead to obvious scams. Facebook, Microsoft, Yahoo etc all suffer the same problems. I think these problems are likely unavoidable at that kind of scale. But I would never rely on these companies as the only (or even primary) line of defense.

Of course the primary line of defense is not running random crap executables.

I'd also recommend uBlock Origin or similar. The number of fake download links you see otherwise is scary.

You need an antivirus that can watch running programs for bad behavior. Polymorphic viruses have been around for decades and will defeat any simple blacklist. And the halting problem means you can't possibly categorize every program as being harmful or not by static analysis.