[geraldwhen]

Reddit bans for linking out to alternative communities. Facebook blocks links to sites it finds problematic, even in private message.

[int_19h]

I remember there was one point when Facebook was blocking links to minds.com.

However, that was quickly explained away as automated spam filter false positive when it was publicized. So even if it happened before, I can't think of any case other than Twitter right now when it was so open and explicit.