Facebook has recently changed its security setting and by default it has disabled secured browsing. You can check whether secure browsing is enabled for a Facebook account URL displayed in the address bar (www.facebook.com – without https).
Enable Secure Browsing in Facebook
Users can turn on the secured browsing by modifying the security setting available under Facebook Account Settings.
Click the Account menu and select Account Settings from the displayed menu list. In the Account Settings screen, navigate to the left sidebar and click the Security link.
Under the Security Settings screen, navigate to Secure browsing section and mark the checkbox labelled as Browse Facebook on a secure connection (https) when possible.
Click the Save Changes button to confirm and save the changes made to Secure Browsing.