pictures look very different at 1000 pixels v 3500 pixels

Could you please post screenshots taken at View > 100% with the pertinent Panels (Toolbar, Layers, Options Bar, …) visible? 

Did you scale the layered image (possibly with Smart Filters) or a flattened copy? 

There are some potential reasons :

First of all when working in Photoshop make sure that you are showing the proper color space (usually sRGB for web)

Then for the export JPEG file format is destructive, so it will alter the images even whan you choose maximum quality. It is way better to export as PNG even if the file is heavier.

Also if you crop an image make sure you do not upsample it. For instance if you crop to half a 3000 pix wide image with an output of 1800 px there will be upscaling and alteration. In this case do not exceed 1500.

And of course many social networks are resampling the images you are uploading, and you cannot do anything about that

And finally the way a browser dispaly ilages may vary from one system to another.

… Hope it helps