I'm guessing they're using watch.ly. I've never heard of the service but it's on the header of the fastcomments blog. Watch.ly mentions a service that will "add realtime hit counters to my products."
Watch.ly is like mixpanel + instabot. I just haven't polished it enough to market it...
FastComments doesn't depend on Watch.ly - it's just there for some UI stuff. Also, Watch.ly sends me a daily email saying what pages (or app screens on native apps) were visited most, for how long, etc, so we can see daily if all the sudden people stop spending time on an important page.
It also gives me a realtime traffic graph. I can see people going from one page, to another, and where they drop off.
