if you lived here you'd be tweeting already
Eric Fischer took a set of geotagged Flickr & Twitter posts, mapped them, and then compared. Flickr is orange, Twitter is blue:
Here are close-ups of San Fransisco & New York:
In these cities, it's clearly not an issue of how much penetration each service has: they're both really popular. But the geographies don't overlap because the places people take pictures of aren't the places they tweet from. That is, people take pictures of tourist destinations; people tweet from where they live. If you were, say, Italo Calvino or Jorge Borges, you could wax philosophical about the astonishing poetry of what we're looking at: it's no less than a visual map of how differently cities look when you view them through the eyes of a tourist or a native.