How "big" are the images you are uploading, that is, what are the pixel dimensions?
I ask because I typically upload images of two different dimensions. My Web "host" is one that I have over the years used for POTN posts, so I upload pics with the POTN-compliant dimensions (1024 pixels at the longest, although for horizontally-oriented pics I'll make it I believe 900 pixels at the longest...
My Web site (PBase) accepts and keeps the "original" but then also resizes the pics to three sizes, Small, Medium and Large, the "Large" being 800 pixels at the widest, I believe.
The thing that is important to me is that I can retrieve a copy of the original at my original resolution for posting. I don't have a complaint, and I never look at my smaller versions to compare.
Now FaceBook is a bit different, and Flickr is a bit different.
I'm not a Flickr user, but I know that if you are a paid "pro" user you can upload a full-rez image and they will keep it and make it available for viewing, the viewer chooses. I've seen good full-size images with Flickr, I typically go right there if viewing if I really want to appreciate the image.
FaceBook has implemented something interesting in recent months! They have started a "Full Screen View" mode in which, if you are viewing an image with a resolution that is equal or greater than your viewing screen you can choose to view in the "Full Screen View" mode and you will see that image fill your screen (without that pesky comments box getting in the way.
What this means is that you can upload fairly large images (larger than our POTN limits, up to what you would consider a "nice" resolution/viewing size. FaceBook does have some "levels" so they will likely resize a bit smaller, especially if they have to fit a smaller screen, but still, it is something to play with, and I've been quite happy with the results. I resize mine when i Export them from Lightroom to be very close to the viewing resolution/size if my "working" monitor, currently a fairly small and inexpensive one with a resolution of about 1600x1050 pixels, so when I view an image in Full Screen mode it nicely fills my screen! People with larger hi-res monitors will just see the "original" size (not enlarged), people with smaller screens will see a down-sized image. Also, if you don't go to the Full Screen mode you tend to see a down-sized image, one big reason why I point people to the Full Screen mode!
And, over the months since this feature has come out, I've tried to let people know if I do post a pic on FaceBook that "It's best if you view this pic in the Full Screen View mode!" and, over the months I've seen others pick up on that practice. And you know what, it can make a BIG difference in the viewing of your image!