I am implementing FB  Login so i Downloaded the SDK from https://developers.facebook.com/docs/ios.
then i drag n down FBSDKCoreKit, FBSDKLoginKit a         
        
For Xcode 7.3, the below worked for me. Follow the steps mentioned on FB's guide and additionally do the below:
step 1. go to Document/FacebookSDK folder
step 2. Click FBSDKCoreKit.framework
step 3. In this folder Remove Modules folder and then drag and drop in your Xcode Project.
References : https://stackoverflow.com/a/29532202
Thanks