I'm pretty sure that this problem may be related to the ATS on iOS 9.
Make sure you have followed the steps provided by the FB team in order to get their SDK working on iOS 9. Release Notes.
EXPLANATION
App Transport Security
"App Transport Security is a feature that improves the security of connections between an app and web services. The feature consists of default connection requirements that conform to best practices for secure connections. Apps can override this default behaviour and turn off transport security."
"All connections using the NSURLConnection, CFURL, or NSURLSession APIs use App Transport Security default behaviour in apps built for iOS 9.0 or later, and OS X v10.11 or later. Connections that do not follow the requirements will fail."
Source: Apple - App Transport Security Technote
This means that if you attempt to perform a connection on an app built for iOS 9.0 or later, or OS X v10.11 or later, and that connection doesn't conform to the requirements of ATS, the connection will fail.
Solution
There are two solutions for this problem.
Recommended
Make sure your connections meet the requirements imposed by ATS.
"Temporary Solution"
Disable the ATS on your app to allow connections that doesn't conform to this.
This may be a temporary solution because as you may now disable the ATS from being used in your app, in future releases this option may be removed and you will be forced to use ATS as a default security feature.
FACEBOOK SDK
Facebook in order to win some time and allow their user to continue using their SDK in iOS 9.0 and OS X 10.11. They picked the "Temporary Solution" and because of that you have to.
- Disable ATS from your app.
- or Whitelist Facebook Servers for Network Requests.
To do the later you must add the following to your target .plist
<key>NSAppTransportSecurity</key>
<dict>
<key>NSExceptionDomains</key>
<dict>
<key>facebook.com</key>
<dict>
<key>NSIncludesSubdomains</key>
<true/>
<key>NSThirdPartyExceptionRequiresForwardSecrecy</key>
<false/>
</dict>
<key>fbcdn.net</key>
<dict>
<key>NSIncludesSubdomains</key>
<true/>
<key>NSThirdPartyExceptionRequiresForwardSecrecy</key>
<false/>
</dict>
<key>akamaihd.net</key>
<dict>
<key>NSIncludesSubdomains</key>
<true/>
<key>NSThirdPartyExceptionRequiresForwardSecrecy</key>
<false/>
</dict>
</dict>
</dict>
Also you may need to perform other changes If you use any of the Facebook dialogs (e.g., Login, Share, App Invites, etc.) that can perform an app switch to Facebook apps.
Source: Facebook - Preparing Your Apps for iOS9