Alt text on Instagram/Facebook vs. alt text for Google

Instagram has an "Alt text" field when you edit a post's accessibility settings. Facebook has one too. Google Images reads the alt attribute on an <img> tag on a web page. All three are called the same thing, and it's easy to assume writing one description covers all of them. It doesn't – they exist for different audiences, get read by different systems, and reward different kinds of writing.

What alt text does on Instagram and Facebook

On both platforms, alt text exists primarily for accessibility: screen readers announce it to people who can't see the image, exactly as intended. Meta also uses it (along with automatically generated object-recognition tags) inside the platform's own recommendation and search systems – its own internal signal for "what's in this photo," used within Instagram/Facebook, not sent anywhere external. That's a meaningfully different job than describing a photo to Google's crawler: it's read by Meta's systems, for Meta's surfaces, not indexed the way a web page is.

What alt text does for Google Image Search

This is the one we've written about directly – on a web page you control, alt text is a documented signal Google's crawler uses to understand what an image shows, distinct from surrounding page content and file names. It matters because Google can actually see and index the page it's attached to.

Why they don't transfer cleanly

The practical gap between the two is less about what alt text is and more about what happens around it:

• Indexability. A photo on your own website, with its own URL and surrounding text, is something Google can crawl and rank. A post on Instagram or Facebook mostly isn't – both platforms are built as apps first, largely gate content behind a login, and aren't structured for Google to index deeply the way a website is. Alt text on a social post is real and useful, but it's working inside a much smaller, platform-controlled space, not the open web.
• Audience and tone. Platform alt text is written for someone using a screen reader while scrolling a feed – short, concrete, present-tense works well ("Golden retriever puppy running on grass"). Alt text for a web page can be exactly the same in spirit, but it also sits next to page copy Google reads together with it, so it benefits from matching the surrounding context rather than standing completely alone.
• Length and format habits. Platforms tend to auto-truncate or nudge toward brevity in their UI. There's no such constraint from Google – the guidance is simply "accurate and specific," not "short."

What this means in practice

Write accessibility-first alt text everywhere – that's the one goal that's genuinely shared across every platform, since a screen reader user deserves the same accurate description regardless of where a photo is posted. Where the effort should differ is expectation: alt text on Instagram or Facebook helps the people using those platforms right now, but it isn't building up your presence in Google Image Search the way alt text on your own website does. If getting found on Google is the goal, the photos that matter for that are the ones on pages you control – your site, your listings, your portfolio – not the copies sitting in a social feed.

This is also why TagDuck's generated alt text is written for that context: a description meant to go on a web page, matched to how Google actually reads images, not a caption meant for a feed. If you're also posting the same photo to Instagram or Facebook, that description is a solid starting point – but it's worth trimming it to fit the shorter, scroll-while-listening context those platforms are actually read in.