My understanding here is that if you're on any social network already at all, you have a bigger problem with this than you would on a Federated Instance. Cause you'll only see what you sub to, and you have granular, person by person choice of who you sub to; moreoever, you could join an instance that had a set of rules that fit rules that were legal/best practice for you, and lock yourself down to that if you wanted. But I don't see how it's any different than being on Facebook or Twitter--or rather, it's safer than being on Facebook or twitter. The idea of federation is that each instance is only 150-200 people, so it's Dunbar's rule in action--an easy group to moderate. I've been told that it's on Tumblr and Twitter etc. that you're likely to stumble on something that's NSFW/illegal/a problem, because there's no abuse or moderation on those enormous sites; and of course, people who are into seriously illegal stuff are often on servers where they are locked down themselves and don't want to risk interoperating with anyone who could report them. So I don't see how you're more likely to have, say, bestiality or violence on a federated instance than through a random Tumblr tag?