Twitter's Visual Insights team has gone and done something awesome. They've created some maps of a bunch of major cities, countries, and continents using geotagged tweets - using only geotagged tweets.
Everything you see in the maps below are tweets - every dot is a tweet and the color of the dot is the tweet count. Twitter says that these maps make use of every single geotagged tweet since 2009 (billions and billions of tweets).
What you'll notice is that you can totally recognize these cities and other areas of the world based solely on boundaries created by tweets.
Look at Europe, for instance:
Here are some more:
New York City
There are a bunch more over on Twitter's Flickr page. I've intentionally failed to disclose the identity of the location mapped out in the lead image. I'll let you see if you can figure it out.