You should know by now that Windows Phone "Mango" has Facebook Chat built in to the Messaging app (see our full Mango review). Turning it on is actually pretty simple: navigate to the Messaging (SMS) app, hit Setting and enable. Or you can do it under Settings --> Applications --> Messaging. Easy enough, so why the how to? Well, this is more a "troubleshooting" thing i.e. you're one of the very few who can't it to work.
See, since I've been running "Mango" (nearly a month next week), I could not get Facebook Chat enabled. Yup. Every time I'd throw that lil' switch, it would ask me to setup Facebook Chat, it would then open what looks like an embedded IE window where it would return with a "Page cannot be found". Seriously. It did this dozens of times and I just thought "welp, it's not enabled yet".
Long story short: I didn't have my Facebook account linked to my Windows Live one. Who knew it was necessary? It would have been nice for an error message to explain that but alas, I was lost till I ran across Mobility Minded's excellent step by step guide on setting this up. The guide was perfect right up to the point when I had to enable the feature and bam...it still didn't work.
The solution was an extra step not in Mobility Minded's otherwise great write up: delete the Facebook account on the phone and set it up again (just name and password). The next time I headed to Messaging --> Settings, Facebook Chat was magically enabled and all is good. Of course the funny part in all of this is I don't use Facebook or Chat period, except for testing. Still, this issue was nagging me and hopefully if any of you have something similar, it'll help too.