The struggle is real…you spend hours editing and perfecting your images only to see them mangled into a blurry pixelated mess when you share them on Facebook. As frustrating as that is, I get it…Facebook has nearly 2 billion users, all uploading photos of their latest lunch, their puppies and kitties and that thing they just saw at Wal-Mart…the amount of space that takes up is astronomical. I don’t even want to think about it! So they HAVE to compress all the joy out of your carefully crafted photos.
There are ways to minimize the damage though, by preparing them yourself to stay within guidelines that keep the evil FB compression bots from squashing your files.
https://www.facebook.com/help/266520536764594/
That link is to Facebook’s help page for uploading images. If you read it carefully there are a few key points to remember: Pixel size, File size and colorspace. For the best possible results I use the larger pixel dimensions. Here’s the export dialog in Lightroom as I set it up for Facebook images:

File settings in the Lightroom Export for uploading to Facebook.
You can see I’ve set the file type to JPEG (I know a lot of sites recommend using PNG, and I do that as well, but I’ve honestly never noticed much difference between the two…other settings are more important, in my opinion.)
Color Space – I always keep this set at sRGB. If you’ve ever noticed color shifts between your computer and your phone and someone else’s computer, this is due to both the calibration of those screens and the color space shifts that occur. the sRGB setting here helps to minimize those shifts.
File Size – Since Facebook recommends keeping the file size below 100K, this is the easiest way to make sure you stay within that boundary. Check the box next to “Limit file size to:” and enter 100 K. This is probably the single most important step to making sure those evil squash-bots leave your photo alone.
Image size – Here is where you have the option to set the pixel dimensions of your image. Typically, as mentioned above I like to have as much detail as possible, so use the highest recommended number from their list: 2048. Set the “resize to fit” options to Long Edge (so the longest side of your photo will be sized to 2048 pixels, and the short size will be adjusted proportionally). Enter 2048 pixels and set the resolution to 72 pixels per inch.
Image Sharpening – this final step is just a little extra punch. I typically have already sharpened my images, sop for this I keep it set to a low setting for screen.
Aside from this, the rest of the settings on your export are up to you. Save the image somewhere appropriate, apply a watermark if desired and then feed that image up to the Mighty Facebook!!!
For reference – there are options within Lightroom for publishing directly to sites like Facebook. Setting up a publish service is a post for another day…
Enjoy, and let me know in comments if there are any questions, or if you notice the bots may have changed and are attacking your photos again…