When exporting a batch of photos in LR you get a variety of options. For photos that are going to be eventually printed I would think that the highest resolution and 300ppi would be a good starting point. But whatabout exporting photos that you know are never going to be destined for more then some nightclubs's website or someone's Facebook? Is there an 'optimum' or most commonly used resolution/ppi/sharpening combination?
When it comes to Sharpening settings I would say it comes down to personal preference. I do not use the tool for Output Sharpening when exporting, any sharpening needed I do in the Develop module.
As for resolution the web browsing "standard" is 72dpi (ppi). But keep in mind that this number does not really matter. In almost all cases of viewing a picture on the screen it is the dimensions that decide, not the resolution. On a printer the final size of a print are the dimensions split by the resolution, not on the screen.
A 300x300 pixel picture with 300DPI will be 1x1 inch when printed, but it will still view as a 300x300 pixel picture in your browser. A good example of this can be found at http://www.scantips.com/no72dpi.html#1. How big the picture is on your screen depends on the screen, they vary in pixel density and size among other things.
My suggestion is that you use the the "Resize to fit" tool, I use "Long Edge" and set it to something that will make sense for the viewer. If the picture is to be in a gallery I tend to stick with 500 to 1000 pixels for the long edge. If it is for phone browsing keep it low. Then I set the resolution to 72 simply because it's the common way.