It is time-based. The globe rotates so that you see the 'edge' when people are starting to tweet.

But people do say 'good morning' at strange times. I suspect this is a mix of people who are either not at their stated location or are travelling, people who are saying good morning to other people (I specifically removed @ replies but I'm sure it didn't get all of these).

I'll render out a static version with a transparent globe - you can see the wave fairly clearly.

Nice Jer. it would be cool to add the day and night shading as a layer to the animation
timeanddate.com/worldclock/sunearth.html

that is really cool, i love it. actually greattt.