I export for the web at 1000 pixels wide (no specified height) and 72 dpi. When opening the exported file in Photoshop, it shows up smaller at 100% view. Something like half the size. If I zoom in one stop, it is obviously blurry.
dpi (I think you mean ppi) is meaningless here. Let's not talk about it any further.
When it shows up in Photoshop, what are the pixel dimensions (width and height) in Photoshop?
Also, what are the (cropped) pixel dimensions (width and height) in Lightroom before you export?
in photoshop, it says it is 1000 x 667 - which is of course what i want but not what is showing up.
the original file is 5760 × 3840
Well this is a guess, but what happens if you export by setting the export dialog box to say "Longest Edge" at 1000 pixels, instead of "Width and Height" with width 1000 and height blank.
Exactly what was the update to your Macbook Pro? Did you switch to a retina screen?
My guess is that you have a Retina screen, and that the horizontal reolution is 2880 pixels.
Most applications compensate for the high pixel density by enlarging everything to 200%, but Photoshop does what it should do, and displays one screen pixel for one image pixel. Although this is confusing, it's what happens when you use a Retina screen.
There have been quite a few discussions about this in the Photoshop forum, that you might want to take a look at.
yes! I switched to a new macbook pro which was my first retina screen. This makes sense! Is there a solution? Should I just be exporting a larger size to compensate?
You could try to export at double size, and see what happens on the web site, it all depends on how the site works. It could be that it will downsize large images in which case they could look better.
The real solution is called @2x images, but it would of course depend on the website catering for it.
In short, you upload two versions of every image, and serve the large (@2x) image to Retina screens.The web browser is tricked into thinking that the larger image (double size) has already been scaled up, and doesn't scale it, so that the image appears sharp and at the right size.
You can read about it here Retina Images - this is for web developers, but might help you understand the issue better.