I don't handle social media so this is only a guess. Does facebook block all referral links? You could try sending a PM to freebitco.in PR.
They don't block, unless you spam too much, usually admins of pages doing it. Though, few years ago when I actively promoted ref links, I wasn't able to post Freebitco.in link on Facebook because it was blacklisted. I had to use shortlinks to bypass it.