There's various bugs open for different implementations of sharing protocols, such as:

• Schema.org - https://schema.org/
  • T106650: Enrich articles with schema.org metadata
  • T106651: Enrich Wikimedia email with Schema.org metadata
  • E.g. Google+ structured data - https://developers.google.com/search/docs/guides/intro-structured-data
• Open Graph - http://opengraphprotocol.org/
  • See: T142243: [EPIC] Add open graph meta tags for subtakss
  • E.g. Facebook Open Graph - https://developers.facebook.com/docs/opengraph/
  • https://www.mediawiki.org/wiki/Extension:WikiSEO exists
• ?
  • T32113: When sharing articles without images on social sites (facebook, twitter, google+) the mediawiki logo is used in lieu of site logo
  • T101711: Banners are used as the illustration when shared on social media websites such as Facebook
• Twitter Cards - https://dev.twitter.com/docs/cards
  • lowest priority, per extensive discussion in T64811: Review and deploy TwitterCards extension for proper display
  • plus twitter uses Open Graph as a fallback for the 3 main items, cf. https://dev.twitter.com/cards/markup

There are also various issues with either implementations of those, or general problems to solve in this area.

See also:

• https://www.mediawiki.org/wiki/Extension:HTML_Tags (no phabricator project)
• https://www.mediawiki.org/wiki/Extension:OpenGraphMeta (no phabricator project)
• https://www.mediawiki.org/wiki/Category:Search_engine_optimization_extensions
• Brief explanations of the differences between opengraph and schema.org: http://stackoverflow.com/questions/6402528/opengraph-or-schema-org )