It’s not really comparable though. The EU isn’t a unified single language market, and its GDP and per capita GDP are much smaller.

Language is not a huge deal - if the French and the Spanish and the Dutch can use Facebook, they could use Eurobook if that existed, as well. The problem of course would be, if they made a committee to build Eurobook, they'd spend 5 years in meetings to ensure every country and every language is absolutely equally represented and then would build something that no speaker of any language would use.

As for GDP, EU overall GDP is only slightly less than US GDP, so it could very well sustain the industry of comparable size. Per capita GDP is indeed lower, but I'm not sure how that precludes creation of something like Eurobook.