I believe that in Facebook you can find a link for your events that you can then add to Google Calendar as something it subscribes to. I've not done this before, but I imagine it's in ical/ics format so it's a common format.

I don't know about the other direction, from Google Cal to Facebook.

To then get the Google Calendar onto your concrete5 site you can embed the calendar through an HTML block.

Google Calendar also outputs a ics feed, which you can read and process - but that's a pretty involved programming activity, where you need libraries, etc, so not really worth going into here.

I've been looking all over for that link, and so far I can't even find a calendar for Facebook events.

I know how to link/embed gcal to the website, but FB is the big hangup here.