This recent study used geotagged tweets to measure social connectedness within American cities. There are two measures: (1) concentrated mobility and (2) equitable mobility. The first measures the extent to which social connections (geotagged tweets) are concentrated in a set of places within the city. And the second looks at the degree in which people move between neighborhoods in roughly similar proportions. These measures are the y-axis and the x-axis, respectively, in this graph:

So how do you read this chart?

Well if you look at New York, you'll see that it is relatively high in concentrated mobility, but the lowest in terms of equitable mobility. This means that social connections are highly concentrated and that there's low connectedness to other neighborhoods within the city. Miami, on the other hand, is the opposite. It's also an outlier. Few hubs. But its social connections appear to cross neighborhoods and spread across the city.

Perhaps not surprisingly, the study found that the size of a city seems to have the biggest impact on social connectedness. Which makes sense -- it becomes harder to get around and so people start to localize. I am reminded of this whenever my friends in Los Angeles tell me they never go to the beach because it's simply too difficult and too time consuming to get across the city.