S Hetjens

This plug-in adds the post thumbnail of articles to the RSS and Atom feed via Media RSS (MRSS) specification. Media RSS was 
invented by Yahoo as an extension to RSS feeds used for syndicating multimedia files (audio, video, image). This 
plug-in adds the post thumbnail defined by author of the article to RSS and Atom feeds of the blog. This plug-in 
activates the post thumbnail feature of WordPress, if the theme is not doing it. It is enough to activate the plug-in 
to work, it has no configuration parameters.
MediaRSS is a nice addition to your blog if you want to syndicate your content to other platforms. More and more 
platforms and RSS feed readers support it. The Notes App of Facebook recognize the post thumbnail and displays it in 
every newsfeed item.

Hetjens MediaRSS

This plug-in adds the post thumbnail of articles to the RSS and Atom-Feed via Media RSS specification.

Matthias Pfefferle

ActivityStreams (<a href="http://www.activitystrea.ms" rel="nofollow ugc">activitystrea.ms</a>) support for your WordPress-blog
ActivityStreams is an open format specification for activity stream protocols, which are used to syndicate activities taken in social web applications and services, similar to those in Facebook’s Newsfeed, FriendFeed, the Movable Type Action Streams plugin, etc.

ActivityStream extension

ActivityStrea.ms feeds for WordPress (Atom and JSON(-LD))

Hetjens MediaRSS vs ActivityStream extension