If you wish to unlink your Facebook account, you can do so in the Main Menu. Step-by-step instructions are available here: Unlink Facebook Account
KNOWN BUG: We are aware of this issue on iOS devices and are working on a solution. WORKAROUNDS: Force quit the app and restart it. Occurs more frequently if left on a in-progress game, so going back to the main screen may help, or close the app when not playing.
Was your word on all neutral letters? If so, that's why! You only score points when your word is connected to a letter in a block you already control. Words played on neutral and enemy blocks will all turn neutral, reducing your opponent's score, but not gaining you any blocks.
See are page Play Your Friends for detailed instructions on starting games with your friends or random players.
KNOWN BUG: This seems to happen if you've had the game running for a long time, possibly days. If you see this, you will need to logout and log back in.