Why do posts to Facebook, Google+, etc. use a different thumbnail and description from the one I provided?

ContentMX Knowledge Base, Using the Platform

You may discover that when you post a web link to social media sites like Facebook, Twitter and Google+ that the thumbnail and description displayed on these sites is different from the one you specified.  This happens because these social media sites (most recently Facebook) are working to combat misleading links and “fake” news. The social media sites look at the web link you are providing and actually go to that page to retrieve the thumbnail and description. In this case, the thumbnail and description are pre-determined and are not in our control.

The platform tries to show you what thumbnails are available and the most likely thumbnail and description that will be used. What will actually be published will be determined by the destination.

There is, however, another way to publish a link with a picture and description you provide. This will not appear the same as when sharing a link, but for some situations it can be effective. This is done by posting a Picture that you upload to the system. The picture should be uploaded to the server (either from your computer or by retrieving it from another site) and added to your Media Library. Then, when you create a new content item to post, select the Picture type and select the photo from the library. In the status field (short or long version), paste the web link at the end of your descriptive text. When this item is posted, your photo will be sent to the social media site along with the description and link and will remain intact when displayed in the newsfeed.