To state the obvious: I did not claim it always works. It is one method of many they may use to check/double-check cases which are otherwise fine, or borderline. I could speculate, but the usage may be more useful in one direction (finding contradictory information) than another (confirming info).To state the obvious, this doesn't always work. My own personal case is one. I have a super common name, and you can find folks with my name all around the world. Meanwhile, I deliberately keep a low profile online and and avoid social media owned by the big companies etc. So someone trying this on me would have to work really hard to filter out a bunch of false positives/false matches only to come up empty in the end. But I'm perhaps unusual in this regard.
The point is not that they ALWAYS use it, nor that not finding someone (those w/o social media profiles) is a big deal. Just that it may complement other methods in useful ways some of the time.
More useful to them than getting 250 page 'books' of repetitive information.
I think we can dismiss as not really relevant the contrived and fantastical.On the other hand, I can think of some contrived exceptional cases where many proofs are required but the applicant is entirely honest and forthcoming. (For example, someone who had the misfortune to be kidnapped and smuggled out of Canada against their will - bypassing the usual entry/exit controls - repeatedly.)