PC or Mac makes a big difference, too. Typically if what you see is too dark your on a PC and the posting party is on Mac. The opposite if your on a mac, the PC images is too bright. It has to do with native Gamma of the sytem/monitor
One needs to get the best calibration possible, of course, however there are many other factors after that.
*Many VFX-film-compositing people use a "dark surround" environment. Dark like going to the movies.
*People who do print work usally a brighter environment
*The color temperature & nrightness of the lights in your work area even paint on the walls can change the brightness and color of what you see.
*How well adjusted the actual image is. Blacks actually being black and whites being whites....A well adjusted histogram (levels or curves in PS for example)
We all do our best to get images to display correctly on the web. It takes work and expereince to get it right, imho. I check my images everywhere I can on other systems, displays... in an ongoing attempt to make sure I'm putting out in the "sweet spot" for all. The best I can do

For the images that don't display right, normally save them and open in PS, check the histogram-adjust the levels so I can determine if it's a display issue or a less than desirable image. That's how you know and many times the only way to actaully see the image LOL Conversely I will view my images in my web browser and make adjustments based on that display, not what I see in PS....working in color spaces and monitor calibration still hurts my brain, then getting images to 'print' the same color....aarrrrgggggg. To be honest, I'm calibrated to print and make 'certian' adjustments for the web LOL
WDA