During the Snowden Disclosures, I took to Facebook with the documents getting published and started to do my own summaries.
Facebook implemented a policy in which it would not let any post that contained a link to a Snowden Document. In some cases they would let the content get posted, but I confirmed with a friend's account that it was not visible when scrolling my feed from another account. Posts about other content were all visible and otherwise had no problems.
I complained and Facebook got back to me with a customer relations paragraph about keeping their platform free of unwanted or questionable content.
Not long after I discovered through media, and then some friends from Turkey, that what I experienced was identical to state sponsored censorship that Facebook extends to the Turkish government.
This lasted for close to the entire duration of public attention on the global and domestic surveillance scandal.
(Other links and content in other scenarios, for example May Day protest organization, was similarly blocked by Facebook - at least in Seattle for the organizers that I was in contact with).