Facebook and Youtube actively discourage external links (to anything) in posts/comments, there are also settings on YT where the channel owner can choose whether or not to allow external links (not sure whether FB allows community mods to decide this or applies its own automatic policies as I've never used it). I noticed this when trying to comment on YouTube with a link to the Dutch public news service; in this case the comment simply disappeared without any warning >>
@VictorVenema @yhancik
I suspect the corporate social networks aren't singling out Mastodon as such, but discouraging *any* site that could lead the discussion elsewhere (as they are losing out on traffic and potential viewers of ads), a link to an old style forum about pets, cars, electronics would be quite likely to also trigger this (even on those forums if you make lots of links in your first few posts they are usually flagged up for the moderators to check)