Facebook data used to identify cities that attract migrants in a systematic way

Systematic migration is defined in the study as “coordinated” migration in these terms:

We formally define a coordinated migration as the movement of population from a hometown h to its most likely current city c(h) such that:

• At least 100 users have listed city h as their hometown.
• At least 20% of the users with hometown h have current city c(h) in their profile (p(h) > 0.2).
• The hometown h and its most likely current city c(h) are different cities. 

To reiterate, what we look into here are those cases where at least 20% of the users from city h moved out AND currently live in city c(h). For this reason, we gave the name of “coordinated” migration from hometown/origin h to current city/destination c(h).