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Seems likely that: (a) manipulating a given person into doing or believing a specific thing they weren't likely to do or believe anyway is actually still very hard, and (b) manipulating enough people by just casting a sufficiently large net works & has worked for a long time.
I think most people believe some version of (a) & (b), where (a) is true if the person isn't gullible & (b) is true as long as the target pop contains enough gullible people. But I don't think gullibility explains much. Advertisers target pops that want what they're selling.
IMO, advertising/meme spread/etc. at any level of efficacy is good or bad for the world only insofar as it's promoting something that's good or bad for the world. Super high-efficacy ads probably create a higher variance spread of possible outcomes, which probably isn't ideal.
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