The force of the constraint is illustrated by the infelicitous character of case (b), but not case (a),
in (20).
(20) a. Have you heard the news about YouTube? It was bought by Google. [acceptable
because the subject is old and by-phrase NP is new]
b. Have you heard the news about Google? #YouTube was bought by it/them. [unacceptable
because the subject (YouTube) is newer in the discourse than the by-phrase]