One key question for Zuckerberg: Will Americans become second-class Web citizens?

BERLIN — Europe had its own successful social media networks once, with obscure names such as SchuelerVZ, Viadeo and Netlog. Almost all of those platforms are now either defunct or have seen their presence largely diminished, as major U.S. tech companies such as Facebook and Google have conquered the market.

Europe is still a market leader in at least one way, though: Its privacy standards are the world’s toughest, especially as lawmakers elsewhere have hesitated or failed to rein in the powerful tech companies.