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Several questions are still unanswered...
Google investigators ran tests over 2 weeks and found up to 496,951 users could be affected. However, that was just 2 weeks. The bug went unfixed for over 2 years. Over the full period, the actual number of affected users could be much higher.
Because Google kept limited activity logs, it couldn’t determine which users may have actually had data taken. But it’s unclear why Google refuses to notify the 496,951 users they believe were at risk.
Google designed APIs to let outside developers collect Google+ profile data of you and your friends, but it wasn’t supposed to collect non-public friend data. Company says it's a “bug” but hasn't gone into specifics on how it went unnoticed so long
Google estimated 438 apps had access to user data. The company tested these apps but did not call or visit with any of the developers, sources say.
Companies may be liable to notify users of a data breach if they know certain types of data was accessed. Because Google didn’t know what data was accessed, it’s unclear what laws may apply.