InDesign export 300dpi to 72dpi

Also the amount you have scaled the image on the InDesign page will affect the JPEG export pixel dimensions. 

If you place your 300ppi, 6000px x 4000px image at 100%, its dimensions in Inches will be 20" x 13.333"—(6000/300 = 20, 4000/300 = 13.333):

The export to JPEG at 72ppi will have a pixel dimension of 1440 x 960—(20"x72 = 1440px, 13.333"x72=960)

If I scale the placed image down, the JPEG Export pixel dimensions will scale accordingly. The placed image scaled down to 25% has an output dimension of 5" x 3.33", so the export to JPEG at 72ppi produces an image that is only 360 x 240 pixels (5"x72=360, 3.33"x72=239.9)

So if I needed to export the scaled image for Instagram, where the recommended pixel dimensions are 1024 x1024, I would export the selected 5" image at 205ppi (1024/5"= 204.8)

Try exporting to PDF and rasterizing in Photoshop. As another user stated, PPI is irrelenant. All images on scren are 1 ppp; that is one pixel per pixel. When you rasterize just worr about using the correct image dimensions in pixels.

The ppi (dpi) is absolutely irrelevant. You need to look at the size in pixels NOT the ppi. Social media has no ppi needs: it could be 72, 300, 3000 or 12.

Your 6000 x 4000 pixel image is beig reduced to 1440 x 960 pixels—the down sample affects image detail