Steps to reproduce:
1. Set up Facebook Developers account and App ID
2. Open QA attached project "PlayerPrefsWebGL.zip"
3. Open "001" scene in "Scenes" folder
4. Set App ID and Upload Access token in Player settings for Facebook platform
5. Build for Facebook platform with WebGL
6. Press "Upload last build to Facebook" or upload it manually in the Facebook API in the browser
7. Open the app on Facebook. Press G to get variable - it will show 0. Press H to set variable and G again, it will show 10. Press S to save Prefs to disk
8. Go back to Unity and build the same project again. Upload it to Facebook and set the new version for production
9. Open the app again and Press G to get variable. It will show 0 again

Expected: If the game URL is the same, the PlayerPrefs should get preserved and upon pressing G in step 9 you should see 10 appear instead of 0
Actual: The PlayerPrefs are discarded

Reproduced on: 5.6.0b1, 5.6.0f1, 5.6.2f1, 5.6.2p2, 2017.1.0b1

Everything post 2017.1.0b2 is unable to run due to corrupted archiving as per Case 926782
No Facebook support in 5.5.*