Google had failed to remove 34% of the scam adverts reported to it, compared with 26% at Facebook, the study indicated.
Both companies said they removed fraudulent adverts, which are banned on their platforms.
But Which? said a more proactive approach was needed.
The report also found:
15% of those surveyed had fallen victim to a scam advert and reported it
of these, 27% had been on Facebook and 19% on Google
43% of victims did not report the scam to the technology companies