If you cannot log into Facebook or if the connection between Sapling and Facebook is temporarily broken (due to an update to Facebook API), you can still log in by entering your username and password. Your username is likely the email associated with your Facebook account, and your password is something that you chose during Sapling registration (not necessarily the same as your Facebook password). If you cannot remember your username and/or password, you can request them.
To request your username or password, click the Forgotten Username or Password? link near the Log in button.