I'm in several photography groups on Facebook, and every single day I see at least one post asking the same thing:
• How do you upload a picture to Facebook and not damage the quality of it?
• Facebook posts are less quality than my original photos. Please help!
• When I upload my pictures to Facebook they look awful!
• What’s the secret to posting sharp images on Facebook?
• Why does Facebook ruin my quality pictures? Are there any tips to make them look better?

Why Does Facebook Ruin Good Photos?

Facebook is a popular place. 2.85 billion people generate a lot of words, a lot of interactions, and a lot of images. On average, about four million gigabytes per day! That's a lot of hard drive space!

Since the vast majority of images that get uploaded are just shared memes and throwaway snapshots, Facebook very wisely compresses them when they get uploaded - it saves them a ton of storage! But for real photographers, this stinks. We spend our money on good gear, our lives learning to shoot quality photos, and our time making each of our photos exactly the right color, sharpness, and depth. So it's pretty disheartening when we put them up on Facebook to show the world only to see how drastically the quality has dropped.

Well, there's no way to keep them pixel-perfect, but there is a way to ensure they are as good as they can possibly be. At the time of this writing (June 2021), this is the straight poop:

Make It the Right Size and Sharpness

Facebook's maximum image size for uploads is a square measuring 2048 pixels on each side. If you upload something taller or wider than 2048 pixels, they will re-size it. That takes the final product out of your hands and puts it in theirs - not a good plan. It's important to adjust a photo's sharpness after resizing it, and I'm assuming that you know how to do this to your photo much better than Facebook's automated process does.

• In Photoshop or Lightroom, resize your image to be 2048 pixels on the longest side.
• Sharpen appropriately

Save It the Right Way

If you don't have an in-depth knowledge of color spaces, your safest bet is to make sure you're always working in sRGB. When you export it from Photoshop or Lightroom, save it as a JPG of very high quality. The JPG format was created to save space. It does this by getting rid of information in your photo. Facebook is going to do that also, so keep as much of that precious image data as you can right now. Set the JPG quality to at least 80 - I always set mine to the full 100.

• Set color space to sRGB
• Save as a JPG, quality of 80-100

Set Your Phone App to HD

A lot more throwaway stuff comes to Facebook via phones. So, in the interest of saving space, the Facebook mobile app compresses images a LOT. Fortunately, there is a "hidden" setting which you can change to keep them from squashing your images so badly. It's called "Upload HD" and you can find it buried in Settings & Privacy:

• Tap the three-line hamburger menu in the Facebook app (bottom right)
• Scroll down and tap "Settings & Privacy"
• Tap "Settings"
• Scroll down to "Media and Contacts"
• Tap "Videos and Photos"
• Under "Photo Settings", tap next to "Upload HD"

Now, DON'T Use the Phone App

Yeah, I know I just told you how to set the phone app to the best quality. Except that's just the best quality available on the phone app. The quality of Facebook's image upload from your computer's browser is measurably better than the HD setting in the mobile app. So if you're a real photographer trying to maintain maximum quality, try to always upload the important stuff from your computer.

• Upload from your computer for better quality than your phone's Facebook app

So, that's pretty much it. The only additional thing I'll say is that you can also upload images as PNG files. I have seen less gradient banding in images uploaded as PNGs instead of JPGs. If your photo has a large area of color with a gradual color change (like the sky usually is), it would be worth your time to do an experiment and see if it looks best as a PNG and not a JPG.