I just read the article, Launching HealthCare.gov from the developers of Jekyll and was definitely impressed with the power that it has to support a site of that scale. Of course there is probably hardware to back up the site itself, but as a CMS, Jekyll can pull it's own weight.

There were a few plugins that they developed in order to support customized behavior, which if you're hosting a Jekyll site on Github then you won't be able to benefit from the use of plugins because Github runs in safe mode, but Jekyll was what is powering the site.

Since Jekyll really compliments my workflow very well, I've definitely become a huge proponent of what it can do. It's exciting to hear about large scale sites using Jekyll, or hearing the unique ways they were able to customize it.