Facebook users who have their Language setting set to English(US) have access to Facebook Graph Search. This is a massively powerful search engine of all the data within Facebook. People, companies, groups and even concepts are represented on Facebook by different types of Pages. Every time a user does something on Facebook it creates a relationship between that user and the other thing they have liked or friended or interacted with. All these pages and the relationships between them are searchable and provide a massive freely available resource to recruiters.