Facebook Bug Bounty
In the middle of January of 2014, I submitted a bug to Facebook through its bug bounty program.
The bug was effectively a Skype account email disclosure. You would find somebodies Skype name, add them on Skype(they didn't have to accept you), and then login to Facebook with your Skype.
Here's a look at how it worked exactly:
In Facebook's "Find Friends" feature, you can login to your Yahoo,Outlook,Skype, and other accounts to add people into your contacts list on facebook(and then in turn add them, I guess).
|The feature in question|
By logging into your skype account on the feature, and pressing "Find Friends", you were submitted to the next page.