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 difficult without intermediaries. We show the increasing role of on-line intermediation platforms in the economy, and their growing responsibility for ensuring global security of people and society. Our contribution is twofold. At a theoretical level, we better define algorithmic intermediation by establishing a hierarchy of intermediation actors, based on their degree of abstraction from specific services. 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 security and that the more abstract they are, the more influence they reach.