The result is usually that you end up digging a bigger and bigger moat around your submission and curation process, so that your service is not completely flooded with crap.
I think this is what you are seeing here, rather than censorship of this specific link or cause.
I once posted a URL to a forum site I was developing for a community to Facebook, and somehow it set off the spam detector. For the next several months, I would get regular notices that a comment I left on another group's discussion was marked as spam, and was not visible to anyone else. I was posting regular comments, without links, apolitical, not critical of anyone, just adding to the discussion.
The notices have since stopped, but I still rarely get any response to my comments, and anything I post to my profile's feed usually gets "ignored" completely, while in the past I'd get a pretty stable number of likes from friends (in the low double digits or high single digits.)
As someone who has moderated a decent size sub-reddit in the past, I understand that there's no malicious intent on Facebook's part in this. They are just so much bigger than this one individual that they simply cannot see me, the same way I cannot see a bacteria, and may not notice a tiny gnat.