Make sure you set the color space on your export to sRGB. Your phone doesn't color manage the images and if you selected any other color space in the export your colors will go bad.
PS also do this on stuff you send to a website. Many folks do not have color management in their browsers and it is not unlikely that your Facebook posts on their computers look just as bad as on your phone.
Make sure that you always set Color space to sRGB when exporting for the web.
However it might not help in this case, I believe phones are in general not color manged, but if you're not using sRGB, it's worth a try.
Are all images dark on your phone, or just the ones you upload yourself?
Is you computer screen calibrated? Maybe you have it set to bright so you are adjusting the photos dark to compensate.