Go to http://farmhack.net/user. If you are logged in you will see your profile page, if not will see a form to log in with your FarmHack.net username and password. If you have forgotten your FarmHack.net username and/or password you can get an automatic login link sent to your email by entering your email address here.
Step 2 - Click on the "Login with Google or Facebook" tab in your profile
Step 3 - Click the "Add linked account" link.
Then you will be prompted for which account you would like to log in with. You then be asked to login with that account... And then your done! You can log into your FarmHack.net account using the Google or Facebook account you added.
The next time you have to log in you can use the Facebook or Google buttons depending on which account you added.