Facebook is alerting its users about a security breach due to a technical glitch, that may have inadvertently exposed the email addresses and telephone numbers of roughly 6 million users. The problem stemmed from a tool that allows users to upload their contact lists or address books to Facebook so that the social network can serve up friend recommendations or invite people to join Facebook. Facebook immediately disabled that tool in order to fix the problem and then turned the DYI back on the next day. Facebook said so far it has no evidence to believe the bug was exploited and has not received complaints from users.
Source: https://thehackernews.com/2013/06/facebook-hack-exposes-contact.html