In truth, there's not much to it because the principle is very similar to that under Windows Phone 8.1. But everything looks different and the settings you need to check are new, so here goes.
Prerequisites:
- patience!
- a smartphone upgraded to, or running, Windows 10 Mobile preferably on Redstone (Anniversary Update) - which currently means on one of the Insider rings but shortly can also include production devices. Results will vary if you're on Threshold - let us know if you're brave enough to try this!
- the Facebook Beta UWP application installed.
Go into Facebook Beta and head for the 'More' control at the bottom right. Swipe up and you'll see a 'Contact and Calendar Sync' function listed:
Tap this and then turn both the Friends and Events links on. Each will immediately try and sync through into your Windows 10 People and Outlook Calendar. The former will take a minute or so, so be patient (thereafter it's much quicker and works in the background).
Start the People app:
In People, tap on the blue hyperlink besides the word 'Show'. This will show your 'Filter contacts' list and there at the bottom is a new option, to include all your Facebook friends. Likewise, in Outlook Calendar, tap on the top left hamburger menu and you should see 'Facebook' in the list of calendars to show.
Pretty cool. My guess is that Facebook will bring this out of beta and over the top of the existing Facebook client at the same time as Microsoft pushes the Anniversry Update to non-Insider devices later this month.