I keep my NEF & sidecar, also the PSD of anything that went through Photoshop.
Any intermediate TIF is discarded [as it's in the NEF+Sidecar]. You can get an 8TB drive for a couple of hundred $£€ these days. Buy a new one when that's full.
I wouldn't compromise the data on my keepers, I'd just learn to cull harder.
I noticed that there was no mention of aspect ratios in the previous answers.
With Instagram, I've found that square or 4x5 (portrait) images tend to get better results because they fit within the platform's window better. Anything else is cropped and users have to go out of their way to view the entire image. There's some speculation that 4x5 images receive ...
None of 1-4 matter. All the major social networks (definitely including Facebook, Twitter and Instagram) re-encode any uploaded photos with their own settings, optimised for their use case (reducing file size/bandwidth) rather than yours. Obviously they can't re-add quality that wasn't there, but they can and will remove any carefully optimised settings you ...
As mentioned in Philip's answer, sites will re-compress to their own standards, but you might as well start off with as good an image as they'll let you upload.
Pixel dimensions, whatever they ask for. DPI/PPI has zero relevance for on-screen display.
png if you need transparency, otherwise jpg.
Related to 1. If they have a file-size limit, set ...