I found the issue: the code in UnityWebapps asks the webview to navigate to m.facebook.com *before* we move the cookies; this makes it so that the cookie jar gets populated with some defalt cookies and then the move fails.
I found the issue: the code in UnityWebapps asks the webview to navigate to m.facebook.com *before* we move the cookies; this makes it so that the cookie jar gets populated with some defalt cookies and then the move fails.