I can try to answer some of that anyway.
I think it depends on what social media platform you're using. If you use one that's primarily visual, my guess is that it does not include any text in the background that people with visual impairments would be able to use. If you're talking about text-based media like Twitter, that probably would not be a problem. People would be able to access it fairly well. I think maybe other people can talk about what it is for them.
It really depends. It's like television. There are certain channels that provide what's needed for people who can't see what's on the screen to understand it. Most of them don't. It's the same with social media. Most of the stuff doesn't.