The map shows the relative probability that a Facebook user in the selected location has a friendship link to a Facebook user in every other location at the same level.
The Social Connectedness Index is the number of Facebook friendship links between two locations divided by the product of their Facebook user populations, scaled so only relative values matter (e.g. if the SCI between location A and location B is twice as large as the SCI between location A and location C, then a randomly chosen Facebook user in location A is twice as likely to be friends with a randomly chosen Facebook user in location B than with a user in location C).
More details, including the full data download, are available at the Humanitarian Data Exchange.