Output depends a lot on where you're putting the photos and the recommendations and restrictions for those sites. For example Facebook (last I checked) recommended 1920px on the long end. Here, if you're above 1048px (?) they will resize and strip the EXIF data (but that may have changed). I am a member of a photo challenge site that requires nothing larger than 1200px on the long side. The local club contests cannot be bigger than 1050px.
I don't export from Lightroom, but when I did I had a number of export presets that I would tune for a specific purpose. The risk of using a single preset is that sites will generally resize on import and you never know what that's going to look like when they're done with the image you worked so hard on.