Facebook Inc.’s advertisers in Europe are on the front lines of its efforts to tighten privacy practices, and their frustrations help explain why the social-media giant’s troubles are far from over.
Facebook said Europe’s new privacy law—General Data Protection Regulation, or GDPR—contributed to slowing revenue growth in its quarterly earnings report on Wednesday, which sent its shares plunging nearly 20% over Thursday and Friday. Some advertisers say another looming privacy change at the Silicon Valley giant could amplify...