Only the pixel dimensions matter for web images, ppi is meaningless in this context. The image will display the same no matter what number ppi is set to.
72 ppi has however become some sort of standard for web images, but this is based an on a misunderstanding - and the number has no practical consequence, except when printing, when it is used to calculate the physical dimensions of the print.
you make a good point. 72dpi does seem to be a legacy value. Actual screen rez varies these days dependant on device, so even "display at 100%" wont be consistent across devices/screens.
I used to use LR Web-module as a simple way to convert raw files down to webfriendly sizes (and it used to be 72dpi)
... but after doing a quick test it seems the latest versions of LR (4,5,CC) all default to 240ppi for the web module.
Feeling very sheepish now for giving the Adobe Tech guys a hard time tonight because i thought the change to 240ppi was something new or a bug. I WAS WRONG!
If you just want to create jpgs, and not galleries, it's easier and faster to use Export instead of the web module.
I do this quite a lot, and have made presets for thumbnails and large images.
FWIW, you can also enter any ppi value you like ...
(and it is ppi - pixels per inch, not dpi - dots per inch, which refers to the resolution of a printer)