Facebook recognizes the login browser as WebView. If you want to prevent this message, change the user agent of your WebView app. In WebViewGold for iOS, you can add this user agent in the Config.swift file in the useragent_iphone and useragent_ipad variables. In WebViewGold for Android, you can insert this user agent in the Config.java file in the USER_AGENT variable. For example, it could be the following user agent:
Mozilla/5.0 (iPhone; CPU iPhone OS like Mac OS X) AppleWebKit/603.1.30 (KHTML, like Gecko) Mobile/14F89 Safari/602.1
More user agents can be found here.