You have two options here you can do this from the IDX Link or within Flexmls.
1.) Bring up the listing you wish to share. Click on the three dots > 'Copy Share Link'
2.) Copy the URL and paste it into facebook
3.) It should display like this:
2.) Click on Facebook (or other social media option available)…
3.) If you are signed into your Facebook account, you will be able to share the listing on your time line after clicking on Share Link…