What does Prosōpobiblium mean in Latin?

Prosōpobiblium, a proper noun, means “Facebook”.

What is the genitive of Prosōpobiblium?

The genitive singular is Prosōpobibliī. Prosōpobiblium is neuter, second declension.

proper noun · neutersecond declension

Prosōpobiblium n sg (genitive Prosōpobibliī or Prosōpobiblī)

Ecclesiastical[pro.s̬o.poˈbiː.bli.um]

declension
form	singular	plural
nominative	Prosōpobiblium	—
genitive	Prosōpobibliī Prosōpobiblī	—
dative	Prosōpobibliō	—
accusative	Prosōpobiblium	—
ablative	Prosōpobibliō	—
vocative	Prosōpobiblium	—

Calque of English Facebook, from Ancient Greek πρόσωπον (prósōpon, “face”) + βιβλίον (biblíon, “book”).