[thirdsun]

I understand that your audience is unlikely to setup an rss reader, but what about an old fashioned newsletter? Build a simple website (if you don't have one already) and let people subscribe to your events/announcements via email. Services like Mailchimp would come in handy to manage subscriptions, create newsletters and, of course, send them.

Also, explain the problem to your facebook users and encourage them to use the email subscription to not miss any future events.