Facebook are an annoying platform and a very centralized one. I don't think they involve themselves with intentionally censoring competitors, but it's absolutely possible that the domain steemit.com might have been blacklisted there due to spam or scam reports. It's quite possible that unfaithful spammy people might have tried to promote their steemit acricles in spammy ways which migth have lead to the domain being burned on there. I haven't checked what's the case and status. But as bad as Facebook are, intentionally doing something like this is too much of a potential liability and possibly grounds for a lawsuit.

Try sharing your articles from the steem blockchain with a link from a different interface like busy.org or something even more obscure (you'll have to search for that) and you might have better luck.