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.
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.
FB was incubated in a single unified market before it really spread to the EU. It’s harder for companies to take off and reach tech giant reach with the much smaller individual markets in the EU.
It’s much harder to build a product that appeals to everyone from the Irish to the Bulgarians, and to advertise to them than it is to do the same for everyone in the US. And it’s not just the tech companies, the individual content creators on the platform have the same comparative problems.