I would like to know if there is a way to automatically invite my Facebook contacts to join Google+.
Here's a workaround for inviting all of your Facebook friends to Google+
The steps are basically:
- Create a Yahoo! account and import your Facebook friends as contacts (you can do this through Yahoo)
- Go to your Google+ page
- Under "Circles" click on "Find People" at the top
- Click "All suggestions" to open the drop down (also at the top)
- Select Yahoo! and follow the instructions to import the contact list from the Yahoo! account in Step 1.
You'll need to do this Yahoo! intermediary step because Google+ cannot access your Facebook friends list.
Inviting them is easy, but forcing them to use Google+ is nearly impossible.
The easiest way is suggested by @OnenOnlyWalter
But from my experience most of the Facebook users rarely open their email account and most of them won't see your email invitation.
Another way to do it is invite them directly on Facebook by setting up an event and then use a 'Select all friends script..' to invite all your friends to the event. Done, they all will get notification about your event, and maybe they will join G+ too, hope so :)