There is an option in the app that does autmatically export the linkings between contacts and friends to your contacts under “Settings” > “Automatic Export”. This will add a field to each contact containing the facebook profile data of the friend.

This means, all linked friends are exported and saved into your contacts. The app will automatically take care to import those friends on each app start and to update them after each change.

Why I use a field in the contacts instead of common ways?

The problem is, that so called ids can (and will) change if you restore your contacts on a new device. To avoid problems because of this, I simply export my data as contact fields and let gmail sync the rest. After restoring contacts on a new device, the contacts on the phone already exist (incl. their new ids) and I can import everything without problems.