Follow these steps in order to enable Facebook chat on your WP7 device with mango:

• Go to http://profile.live.com
• Log in using your PRIMAIRY Live account. This is the account you entered when initially setting up your phone. Find this on your phone by going to Settings / Email+accounts, it’s the account listed as “Windows Live“.
• On the left side, hit manage and connect facebook.
• Grab your phone and go to Settings / Email+accounts, press and hold your Facebook and primiary Live ID and choose Synchronise.
• On your computer, go to facebook.com, log in and scroll all the way down. On the bottom you can change the display language used for facebook, change this to ENGLISH US.
• Grab you phone, go to messaging and enjoy facebook chat!

Please leave a reply whether it worked or not.