Mapbox's Eric Fischer put together a map of where each of the almost 6.5 billion geotagged tweets sent in the past three and a half years came from. What you see below is a gorgeous, interactive depiction of tweet density — the brighter the green splotch on a map, the more tweets that have been from there since mid-2011:
The map is super-versatile: you can look closely at one region or country, or zoom all the way out at the global level. The latter is particularly striking if you think about who lives where. Here's global tweet density:
And here's national population density (in 2010, but still basically useful):
In most places, they look pretty similar — where there are large concentrations of people, there are large concentrations of tweets. China might be the most glaring exception: Twitter is banned there, though it's quite possible to use if you try hard enough. A site called Weibo serves as the government-approved alternative. India and sub-Saharan Africa are also underrepresented.
Correction: An earlier version of this post said the map looked at all tweets sent in the 3.5 years. In fact, it only covers specifically geotagged tweets.