I see that claim thrown around a lot but nothing ever backs that up.
There's a study conducted by Facebook about controlling emotions by means of content curation. If Facebook can do it, Google too can do it: https://www.theguardian.com/technology/2014/jun/29/facebook-...
Google is the dominant search engine and these days they even host the content through AMP and they own the most popular web browser and mobile computers operating system. They have not only the means to reach people and curate the content generated by other people, they can also change the content created by other people(AMP pages) and disguise their actions(since they control the app that displays the content).
Let's travel in time and go back to the WW2 days and mention Hitler as any proper online political discussion would eventually do.
Don't you think that it would be very convenient for Hitler if he had a way to show "how awful Jews are"? Wouldn't he loved it if Germans search results about economical crisis or gas prices would display how "greedy Jewish bankers are destroying the lives of honest German workers" or any other political or cultural Nazi propaganda?
Unless we switch to cryptographically signing all the content we create and learn to distrust any content that's not signed and have a way to verify that the signed content curation is not politically influenced, giants like Google and Facebook would have the power to make/break countries, start/stop lynches or even genocide.
There's a reason why China doesn't allow search results about Tiananmen Square protests of 1989 and all the governments are trying to find a way to control the information flow.
April 2018: We developed all that technology by reading your e-mails and analysing your daily life, now we will make military gear with all the expertise we know about the world and the human population so that political actors can kill more efficiently. No biggie.
PS: We control the information flow, we can start and stop political conflicts by selectively displaying information but we would totally not use it to sell military gear. Oh, we also don't claim to be not evil anymore but no biggie. We also have established political ties and spend tons of money on lobbying but we would definitely not extract wealth by starting political conflicts and selling gear that is designed to contain the specific conflicts to the people in power that we spend a lot of money to keep in touch. No biggie.