When you share a URL on Facebook, it scrapes the page for the URL for the Open Graph data to determine what image/video to display.  Facebook caches the image for that URL, so if you change the image and then share the same URL at a later time, the old image will still get displayed, which is probably not what you want to do.

Facebook provides the Open Graph Debug Tool that can be used to manually clear the cached value for the URL, but that may not be a workable if your website allows images to be changed dynamically.  Ideally, you would want a way to programmatically clear the cache after the associated image is changed.

Fortunately, there is a way to clear the cache using the OpenGraph API by forcing a scrape of the URL. The key is to set the ‘scrape’ parameter to true in the API call. The following Ruby code assumes that the Facebook application id and secret are stored in environment variables.

facebook_open_graph.rb

    module FacebookOpenGraph
      def self.clear_cache(url)
        params = {
          :client_id => ENV['FACEBOOK_APP_ID'],
          :client_secret => ENV['FACEBOOK_SECRET'],
          :grant_type => "client_credentials"
        }
        uri = URI("https://graph.facebook.com/oauth/access_token?#{params.to_query}")
        response = Net::HTTP.get(uri)
        access_token = Rack::Utils.parse_nested_query(response)["access_token"]
        unless access_token.nil?
          uri = URI('https://graph.facebook.com')
          res = Net::HTTP.post_form(uri, 'id' => "#{url}", 'scrape' => 'true',
              'access_token' => "#{access_token}", 'max' => '500')
        end
      end
    end

Then to clear the cache you merely need to call the clear_cache method, passing in the URL being shared.

    FacebookOpenGraph.clear_cache("http://myurl.com/some_cool_page")