On June 2, 2015, Facebook announced that it would stop supporting Facebook-connected apps that were signed with SHA-1, as of October 1, 2015.
“These changes are part of a broader shift in how browsers and web sites encrypt traffic to protect the contents of online communications. Typically, web browsers use a hash function to create a unique fingerprint for a chunk of data or a message. This fingerprint is then digitally signed to prove that a message has not been altered or tampered with when passing through the various servers and systems between your computer and Facebook’s servers.” [https://developers.facebook.com/blog/post/2015/06/02/SHA-2-Updates-Needed/]
In its announcement, Facebook acknowledged that the CA/Browser Forum’s Baseline Requirements for SSL sunset SHA-1-based signatures as of January 1, 2016, but that it would be “updating [its] servers to stop accepting SHA-1 based connections before this final date, on October 1, 2015. After that date, we’ll require apps and sites that connect to Facebook to support the more secure SHA-2 connections.”
Applications, SDKs, and devices that connect to Facebook will all need to support SHA-2, but those that still rely on SHA-1-based certificates will not work with Facebook. The CA Security Council has prepared a whitepaper explaining some of the issues relevant to this transition.