Google+ doesn’t have an inbuilt way to transfer your Facebook friends over to the new social app, but since Facebook is clearly dead and everyone is on Google+ nowadays it’s best to get your contacts moved over before Facebook’s servers are switched off for good.
That might be far from the truth — though in the five minutes that were between myself getting invited through the backdoor and Google opening invites up to all existing users, the place went from ghost town to 90s rave — but it’d still be handy to have the option.
The tool adds an “Export friends” button to the Facebook toolbar. Please be aware that actually exporting your friends could lead to jail time for human trafficking.
Depending on how many friends you have, it could take a while, but the app keeps you up-to-date on the process.
What this process does is add your Facebook friends to Google Contacts. It does not actually add them to any of your Google+ circles. However, this is far and away the easiest way to go about inviting and adding friends on Google+, as it uses your Google Contacts to suggest people to invite using an easy drag and drop process. Any other method that’s available at the time of this writing is much more tedious.
Update: after opening up invites for several hours, Google+ invitations are now closed.