Can we see your facebook and instagram to see how well you've done? Perhaps we could give you ttips. I don't use facebook anymore but hopefully I can still see your page.

Write your main post then just add in a paragraph underneath with the image description in the image caption. Screen readers will get that perfectly. Same goes with instagram, just add a description in the caption. If you plan to do anything with Twitter, when you tweet an image, you'll be given the option to add alt text directly to the uploaded image which only screen readers will see. Those don't have character limits, or at least the character limit for a Twitter image description is over double the main tweet character limit.

I have to say Instagram is really not user friendly with VoiceOver. It's difficult to navigate unless you've still got a decent amount of sight left. That being said, if you did include image descriptions, that'd be really cool.