Emily Badger is a former staff writer at CityLab. Her work has previously appeared in Pacific Standard, GOOD, The Christian Science Monitor, and The New York Times. She lives in the Washington, D.C. area.
Which cities attract the most people, and what kinds of communities are losing them.
Facebook possesses a startlingly massive trove of data on global migration patterns. It's hidden, innocuously enough, amid two simple biographical details that many people post on their profile pages: Where you live, and where you're from.
Between those two data points – spread across the millions of its 1 billion users who volunteer this information – Facebook can paint a picture of where large population shifts take place, which cities seem to attract the most people, and what kinds of communities are losing them.
Facebook's Data Science team recently produced an interesting analysis of what it's calling "coordinated migration" patterns among large groups of people who tend to flock from one city to a common destination (think Warsaw to Chicago, Havana to Miami, or from just about anywhere in Turkey to Istanbul). The results require a grain of salt: Facebook's sample admittedly self-selects for people anywhere in the world who use computers, and as Wired points out, the company's in-house analysis is not exactly open for peer review.
But it's intriguing to see how this otherwise unremarkable biographical information on a social media site can add up to larger narratives about human movement. In fact, Facebook's ability to do this, particularly at the city level, seems to be Facebook's point. As the data science team explains in the conclusion to this analysis: "Facebook offers a wealth of data suitable for the study of human mobility."
In this particular case, Facebook looked at all users who offer up both a hometown and a current location (it doesn't say how many people this represents). The analysis isn't just interested in where people move from a given hometown, but where the largest share of people who grew up there live now.
For instance, let’s say 1,000,000 people list Boston as their hometown on Facebook. Out of these individuals, 300,000 list Boston as their current city, and no other city has more individuals listing Boston as their hometown... it follows that Boston is the most likely current city for people who grew up in Boston. People who grew up in Boston still live there with a 30% probability. This is quite a common occurrence – for many cities, people are most likely to stay where they grew up. The study of coordinated migration focuses on cities for which the most likely current city is different from the hometown. For example, 67% of the individuals with Badagri, Nigeria as hometown have Lagos, Nigeria as their current city. Lagos is the most likely current city for people from Badagri.
This means there are more people – per Facebook – moving from Badagri specifically to Lagos than staying in Badagri.
By this definition, Facebook's conclusion may not surprise you. Rapidly urbanizing cities are attracting people from neighboring communities (typically within the same country). All over Nigeria, people are moving to Lagos. In Ghana, they're moving to Accra. In India, they're moving to Hyderabad and Chennai. These destinations, Facebook points out with the help of World Bank data, are urbanizing at a particularly fast clip.
Each hometown has only one primary destination, but each destination city is fed by many hometowns. Here is what that relationship looks like in Africa, with origin cities in blue dots, and migration destinations in red ones (Facebook appears not to have published the full map from which these screen grabs come):
Here is the gravitational force that is Istanbul:
Western cities don't show the same patterns today: They "attract population from all over the world," Facebook writes, "but rarely in a coordinated way." But we can see a few of these migrations play out across North America, notably from several Mexican cities to Chicago:
In countries like Nigeria and India, the people living in smaller cities or rural towns are less likely to be on Facebook, and that invariably skews these results (people with a hometown of Badagri and a current town of Badagri are no doubt underrepresented). But the patterns are compelling even with their limitations. And if you were curious what Facebook was doing with your data, other than selling ads against it, this is it.
All images courtesy of Facebook.