What does Pariser mean when he says we've moved from human gatekeepers to algorithmic gatekeepers on controlling the flow of information? Why is this a problem?

What need most led to the creation of the advanced research projects agency network? darpa wanted scientists to be able to collaborate and share research easily. darpa wanted american and russian politicians to be able to communicate. darpa wanted astronauts to be able to contact nasa and the white house. darpa wanted factory owners to be able to coordinate supply chains.