This is usually caused by the site, not your RAW file. Services like Facebook and Flickr often resize and recompress uploads, and JPEG compression tends to damage smooth dark areas more visibly, so low-key images can show blockiness or banding after upload.

A few practical ways to reduce it:

• Export at or near the site’s display size so it does less resizing.
• Use export settings tuned for that service and test what holds up best.
• Link to or encourage viewing the original/full-size file when possible.
• Avoid pushing processing so close to the edge that extra compression breaks it.
• In some cases, adding a very small amount of grain/noise/dithering can help dark gradients survive JPEG compression better.

If the service always recompresses aggressively to a fixed target, there may be no complete fix. In that case, the only real solution is to use a platform that preserves image quality better or share the original-size image.