If you routinely upload photos from your phone to Facebook, you have probably noticed that the end result is what you might call potato quality. Facebook compresses all uploads regardless of the source to ease the strain on their servers, but mobile uploads have gotten it worse than those done on desktops. A new setting rolling out, though, allows users to opt into higher quality uploads from the Android app.
I am using the latest version of Facebook pushed to me from the Play Store, 188.8.131.52.59. With that said, the toggle for HD uploads did not appear immediately after I updated, so there may be a server-side switch involved. In any case, the option to upload photos at a higher quality is within the app settings; it is turned off by default.
To see the effect, I used a test photo from my phone at the two different settings. Here's a link to the original on Google Drive. It is 6 MB and 5312 x 2988 pixels.
With HD upload enabled, this is the end result. It is reduced to 443 KB, which is not proof that the quality was reduced since the compression could be simply eliminating redundant information. The dimensions are, however, reduced to 2048 x 1152 pixels. This is still plenty large for most of the reasons you would want to look at photos on Facebook. The metadata reveals this photo uses an sRGB color profile like the original, but a black-scaled version that likely explains some of the file size shrinkage.
Importantly, 2048 pixels is the maximum width you will achieve with desktop uploads, so this option gives you the same treatment non-mobile users get.
For comparison, this is the output with HD upload disabled. The file size is now just 58 KB, a pretty good deal on Facebook's end. The dimensions are a measly 960 x 540, which isn't going to look great on a lot of screens and will be useless if you have any desire to zoom in on it. This file uses a "c2" color profile, likely the more aggressive reduction of color values discussed by Facebook Engineering in this post from four years ago.
Clearly, Facebook isn't about to replace Flickr or some other place as the destination for high-importance photography, but at least you won't have crappy-looking mobile uploads.