Facebook is back on the defensive after revelations about its extensive data sharing partnerships with device makers. Just a few months ago the social media giant assured the public and Congress that developers had been cut off from the kind of data access that had been exploited by the political consulting firm Cambridge Analytica. However it did not disclose that makers of cellphones, tablets and other hardware were exempt from such restrictions. So what does this mean for users and how can we be reduce the amount of data that's kept on us? Serge Egelman is a privacy researcher at the University of California, Berkeley, who studies the security of mobile apps.