Eric Fischer at Mapbox has mapped 6 billion geo-tagged Twitter messages. The 6 Billion Tweets Map is a great visualization of Twitter's global appeal, showing where Twitter is popular and also where in the world the social network has yet to gain traction.
Mapping 6 billion Tweets is also quite a technical achievement. If you are interested in how Eric was able to manage mapping so much data on one map then you should read his Mapbox blog post on how he created the map. In particular you might be interested in a new tool which Eric has developed for making vector tiles from large data sets, called Tippecanoe.
For me one of the most interesting things that the map reveals is that Twitter can't handle Tweets posted along the Prime meridian. A blank band runs vertically down the map along the meridian, so it looks like Twitter's geo-location tools can't handle Tweets with a longitude of 0°. Source