I'm the developer for this site, and we're using Node.js w/Express, MongoDB, and deployed on Heroku. Mongo might not scale super well for this kind of thing, but for ease of development you really can't beat it.
Yeah we found that library posted on HN a few days ago, and figured since we have very low traffic right now we'd have nothing to lose with a library like this (even with the additional requests it sometimes incurs when a user hovers but doesn't click). It's so fast now I'm not sure I could ever bear to build a site without it ever again.