Recent changes by Apple to improve privacy for users of iOS, its mobile operating system, may be affecting tools used by Facebook to help advertisers measure the effectiveness of their ad campaigns, The Wall Street Journal reports. In a blog post, Facebook Vice President of Product Marketing Graham Mudd suggested Facebook may be "underreporting iOS web conversions by approximately 15%." He also said Facebook is "optimistic about our multi-year effort to develop new privacy-enhancing technologies" that will help minimize the amount of personal data that's collected while still allowing for personalized ads. The post also includes a list of actions advertisers can take "while still respecting people's privacy."