Automattic

John Godley

Donncha O Caoimh (a11n)

Eoin Gallagher

Lenny

Future upgrades to Sharedaddy plugin will only be available in <a href="http://jetpack.me/" rel="nofollow ugc">Jetpack</a>.
Jetpack connects your blog to the WordPress.com cloud, <a href="http://jetpack.me/support/" rel="nofollow ugc">enabling awesome features</a>.

Sharedaddy

Future upgrades to Sharedaddy plugin will only be available in Jetpack.

aitor.delapuente

This plugin helps ensure your content has a relevant thumbnail when shared on social sites.
Adds an image_src and og:image meta tags to your header using post thumbnnail (you must have post thumbnails / featured image enabled), first image on post/page content, or a defined default image.

FetenWeb image_src Metatag

This plugin helps ensure your content has a relevant thumbnail when shared on social sites.

Sharedaddy vs FetenWeb image_src Metatag