Alt text is, at a minimum, "here's what you need if for any reason you can't see the picture."

What that means depends on what the image is. If it's a screen-shot of a post on a different platform, that minimum's the platform, any username or date, and the text of the post content. For a meme image joke, it means a brief description of the image.

Good systems also let you add more than the minimum, like the "mouseover" extras on XKCD, but these are extra. And they shouldnt detract from the image itself.

socialmedia

Needs a space in there, my dude. It's still two words.