Hi mdb,
There currently is no option to upload content to social media directly from iCloud. If this is something you'd like to see in the future we'd encourage you to leave feedback here: Product Feedback - Apple
If your issue with this process is the amount of storage you have available on your iPhone here's how to optimize your storage:
"Store your photos and videos in iCloud
Your photos and videos are stored on your device in their original, high-resolution version. This means that they use a lot of space on your iPhone, iPad, and iPod touch. With iCloud Photos, you can make more space available on your device and have your entire collection, everywhere you go. You can learn more here: Set up and use iCloud Photos - Apple Support
Get started with iCloud Photos:
- Tap Settings > [your name] > iCloud > Photos.
- Turn on iCloud Photos.
- Select Optimize iPhone Storage to save space on your device."
You can read the full resource here: Manage your photo and video storage - Apple Support
We hope this helps.
Take care.