Lightroom 4.3 and Windows 8
I justed exported a bunch of holiday trip photos for posting on the web/facebook using LR 4.3 on Win8, and included a watermark. They all looked just fine to me. What font did you use? This sample is using "Brush Script MT" size 12 or 13. The image was exported with a 1600 pixel width.
While testing this I did another export at 1000 pixels, and that version did actually have some jaggy text....
I'm using Kunstler Script Regular. It's got thin lines so I thought that might be contributing and I did try other fonts as well. They all show the same issue, as does the sample you posted here (looks good at the smaller size but the full image definitely is not smooth as it should be).
1024 exported in Windows 7
1024px exported in Windows 8