for example, someone might post on facebook "hey look at this crazy article {link}, isn't this {name} from school?"
and google used this data to show the result
That's one of the theories mentioned in TFA.
The primary source might not be metadata but once the connection is made between the two pieces of content metadata is probably used to evaluate it.
for example, someone might post on facebook "hey look at this crazy article {link}, isn't this {name} from school?"
and google used this data to show the result