Any way you can post a screenshot of the art on the artboard and also the saved image?
After giving this some more thought I have figured out what was happening.
I am a newbie so this probably explains allot :}
First time user of the pencil and sketch I just randomly sketched something and did not pay attention to the transparency of any of the selections I made.
When I viewed the image I had pasted in messenger it was with a white background by default (just as it was in Sketch). For some reason when viewed full size it was with a black background on the viewer.
It impacted the image allot as you can imagine. Actually pretty cool changes!
At the time I had not even thought about the transparency settings of what I created for the first time. So I was surprised to see such a dramatic change of the image when viewing full size.
I tested this by manually changing the backgrounds of the viewer and it confirmed this.
Glad you figured it out. And thanks for explaining in further detail what you were seeing and what the problem was.