in reply to Micr0byte

It depends. On the web an alt text must be concise. Everything that is important, makes sense, is mentioned somewhere in the text, is to be conveyed. but conciseness is first.
Here on social networks, I'd say, completeness is the first and even more important than conciseness. For instance, if you post a meme, describe it even if it's super lengthy. Like: "Three panels from left to right, on the first panel there is a man..." and on and on you go. It's important because *the image* is the unique thing you share, I have to laugh, to think, to be angry or emotional about *the image* itself, without any context basically.
Ask further questions, I'm glad to answer everything.
in reply to ∴7700e6 `Violet`

@0x7700e6 Because if you are reading an article, you generally don't want a huge alt text that would distract you from your reading. Even less you want it for images like logos, avatars, social network badges and so on. Also, both in and out of social networks avoid phrasing like "This is an image depicting..." (I know it's an image, my screen reader tells me about it); "This is the avatar of Jane Doe" ("Jane Doe" is enough).
in reply to André Polykanine

@menelion
If I post a well-known meme, is it OK to just say, for example "the Drake meme, with X then Y" or should I be doing "a four panel meme. On the first row left panel a man reacts negatively to the panel on the right showing X, then on the next row he reacts positively to the panel on the right showing Y".
@0x7700e6 @micr0
in reply to stib

@stib @0x7700e6 I'd do the second, sorry to bother you with that. Because we blindies are kinda... behind the graphic memes. You could probably possibly put a link to a description but rather don't because different clients and different browsers don't allow clicking links in alt, it would be plain text so... unfortunately probably you have to describe, at least for the first time.
in reply to André Polykanine

Oh! Thank you for this: I (wrongly) thought that conciseness on social media would be preferred, except where maybe the post contains only an image, of the image is particularly detailed.

I sometimes also might add a commentary or a quip in my alt text - perhaps in response to the post's text, or to add context, or my own reaction. Is this poor practice, though, do you think?

@stib @0x7700e6 @micr0

in reply to André Polykanine

@menelion @0x7700e6 There are dozens of sites where you can overlay your text on popular images to create memes without having to use an image editing program, I wonder if there are any that come with #AltText pre-generated. Eg. in the "Drake meme" format it would supply the image description and substitute your text for X and Y. If not there's my next billion dollar startup, ready to go.
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André Polykanine

@avuko @stib @0x7700e6 It's helpful for the deaf, at least a rough description so they could probably send the video to a software for captions (I'm not sure but I imagine this is possible). For us blindies it's helpful when the video is super visual, like only music and kitties playing, for example, or a guy/girl is assembling, drawing, painting, knitting something etc., where there are no words.
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André Polykanine

@avuko @stib @0x7700e6 For example, my sighted wife likes to watch videos where a Japanese guy shows small apartments in Japan. He never speaks, only sometimes he adds some subtitles in very simple English, as my wife has just said. Maybe he is ashamed of his English pronunciation, maybe he's simply a shy person, I don't know, but he does amazing videos, but super visual. There ideally audio description or at least a decent alt text would work, especially if you share it for a reason (for example, you liked a particular apartment he was showing).