A lot of people use social media. If you post content to social media, ensure you take care of accessibility. There are no separate guidelines for social media. The same instructions and guidelines apply to social media as they do for websites.
For hashtags, it's important to use the Pascal case. There is a huge difference in how the tags are read to screen reader users. When you use the Pascal case, you hear the actual words. If everything is in lowercase, you might hear something that makes sense or you might not.
A lot of sources mistakenly tell you to use the Camel case, even though they mean the Pascal case. So start every word in your hashtag with a capital letter, for example, #ThisIsMyTag.
Make sure you add alternative texts to all your images on social media. If you are unable to add an actual alternative text, you can describe the image in the post body text.
There are a lot of differences in how to add the alternative text on the platforms, but at least most allow you to add one. Some platforms, for example, Facebook, will add an alternative text automatically. The automated alternative text is better than nothing, but often the quality of the automated alternative texts is not that great.
A lot of platforms allow you to add captions and audio descriptions. If the platform you use doesn't, consider adding the video to a platform that does and then share the link to the video on social media.
But at least always provide as much information as you can in the social media post itself about any media content.