Due to a recent Chrome update, some users have lost the "Log in with Facebook" button. Luckily, the solution to resolve this issue is fairly straightforward.
Open Chrome's main menu -- the three dots to the far right of your browser screen.
Under Privacy and Security, turn off "Send a 'Do Not Track' request with your browsing traffic".
If you are still unable to locate the "Log in With Facebook" button, please contact Support.