The potential of Facebook advertising data for monitoring people fleeing Ukraine to Europe

This work contributes to the discussion on how innovative data can support a fast crisis response. We use operational data from Facebook to gain useful insights on where people fleeing Ukraine following the Russian invasion are likely to be displaced, focusing on the \acl{EU}. In this context, it is extremely important to anticipate where these people are moving so that local and national authorities can better manage challenges related to their reception and integration. By means of the Ukrainian-speaking Monthly Active Users estimates provided by Facebook advertising platform, we analyse the flows of people fleeing the country towards the European Union. At the fifth week since the beginning of the war, our results indicate an increase in the number of Ukrainian-speaking Facebook users in all the EU countries, with Poland registering the highest percentage share ($33\%$) of the overall increase, followed by Germany ($17\%$), and Czechia ($15\%$). We assess the reliability of prewar Facebook estimates by comparison with official statistics on the Ukrainian diaspora, finding a strong correlation between the two data sources (Pearson's $r=0.93$, $p<0.0001$). We then compare our results with data on arrivals in Poland and Hungary reported by the \acs{UNHCR}, and we observe a similarity in their trend. In conclusion, we show how Facebook advertising data could offer timely insights on international mobility during crisis, supporting initiatives aimed at providing humanitarian assistance to the displaced people, as well as local and national authorities to better manage their reception and integration.