The Open Graph protocol was created by Facebook in 2010 to enable any web page to become a rich object in a social graph. When a URL is shared on Facebook, the platform's crawler fetches the page and extracts Open Graph meta tags to construct a rich preview card containing a title, description, image, and other metadata. The protocol was designed to be simple and extensible, using meta tags with property attributes (property="og:title") in the HTML head section. Despite being created by Facebook, Open Graph became a de facto standard adopted by virtually all social platforms, messaging apps, and content aggregators.

The core Open Graph tags define the essential preview card elements. The og:title specifies the title displayed in the share card, og:description provides the summary text, og:image specifies the preview image URL, og:url defines the canonical URL, and og:type classifies the content (website, article, video, music, etc.). Image specifications vary by platform: Facebook recommends 1200x630 pixels with a 1.91:1 aspect ratio, while Twitter prefers a slightly different ratio. Images smaller than platform minimums may result in no image preview or a small thumbnail rather than a large card. The og:image should be an absolute URL accessible to crawlers, and some platforms cache images aggressively, making it difficult to update once fetched.

Twitter extends the Open Graph concept with its own Twitter Card system using meta tags with name attributes (name="twitter:card"). The twitter:card tag specifies the card type: summary (small image with title and description), summary_large_image (large featured image with title and description), player (embedded video or audio player), and app (mobile app install card). When Twitter-specific tags are not present, Twitter falls back to Open Graph tags, so most sites only need to add twitter:card and twitter:site (the site's Twitter handle) alongside their Open Graph tags. Other platforms have their own behaviors: LinkedIn heavily relies on og:image dimensions, Discord uses a combination of Open Graph and oEmbed, and Slack unfurls links using Open Graph data plus its own metadata extraction.

Each platform implements its own crawler and caching strategy, which creates challenges for content creators. Facebook's crawler caches Open Graph data and may not refresh it for days or weeks after the initial fetch. The Facebook Sharing Debugger tool forces a cache refresh and shows exactly what Facebook's crawler sees. Twitter's Card Validator serves a similar purpose. LinkedIn's Post Inspector refreshes its cache. When updating Open Graph tags on a live page, manually clearing each platform's cache is necessary to see the changes reflected in new shares. Some platforms also enforce minimum content requirements: Facebook may reject images below 200x200 pixels, and Twitter requires twitter:card to be present for any card to display.