It's pretty simple actually.
With the efforts of Facebook trying to prevent
the option to export your contacts to Google+
(deleting Apps and solution that do it, updating their
Terms of Use accordingly so no one would be able to export)

They left out one aspect, thanks to a partnership with Yahoo! Mail
You are able to export your Facebook contacts to your Yahoo! Mail.
In your Yahoo! Mail go to Contacts>Import Contacts and there
You'll be able to import your Facebook contacts.
Afterwards simply export the contacts to your Gmail.
Because Google+ is integrated with your Gmail account,
It will simply allow you to add those people to your circles
(and invite them to Google+ if necessary)