Intermediation is the action to match two types of actors (users, clients, services, etc.) in a world with incomplete information, where the matching would have been rather difficult without intermediaries. We show the increasing role of on-line intermediation platforms in the economy, and their growing respon-sibility for ensuring global security of people and so-ciety. Our contribution is twofold. At a theoreti-cal level, we better define algorithmic intermediation by establishing a hierarchy of intermediation actors, based on their degree of abstraction from specific ser-vices. Then, surveying data from web analytics tools such as Alexa.com, we show that there are only few important intermediation platforms, headquartered in few countries. These results underline the strategic importance of intermediation actors in terms of secu-rity and that the more abstract they are, the more influence they reach.