Facebook and other Silicon Valley giants could face more scrutiny and sanctions in the European Union after the bloc's top court decided to back national privacy watchdogs to pursue them, even when they are not their lead regulators.

Consumer lobbying group BEUC welcomed Tuesday's ruling by the EU Court of Justice (CJEU), which backed the right of national agencies to act, citing enforcement bottlenecks.

Along with Google, Twitter, and Apple, Facebook has...