You see, I think when people say "well, we have access and regularly check what they're doing etc" that actually, in reality, it's a bit of a false sense of security - at least that's the conclusion I've come to (when I look at cases where kids have harmed themselves or revealed they've been groomed or abused online).
I say that not because the intent is flawed or stupid or inappropriate by insisting on access - it's because you love your children as I love mine and we want to protect them. I get it 100%.
But even if you monitor FB you don't get to see all the private messaging (that's right isn't it) - so you still don't know what they might be receiving or who or how they're talking to through that. Or if they have gmail/youtube/google+ any freak can email them through youtube for example if the settings are wrong, then there's Snapchat, Instagram, their texting (highly likely someone would ask them for pics or sexting action through SMS), then there's the ability to have separate accounts (ie ones you don't know about possibly), and umpteen apps and so on that probably you and I don't even know about yet cos we're old fuddy duddies (well, I am!) that they're using to chat with one another (aside from facetiming and other non-Apple facetiming versions). Or if they leave up the non-private account(s) but have another account you don't know about somehow...
Just trying to keep up with the changing account privacy settings, permissions settings alone is a massive job across all the platforms they use to keep in touch with each other.
And what about when they're at someone else's having a sleepover or hanging out at a mate's? What's to say that someone's dad or brother isn't a real life in the flesh paedophile - they're not safe bloody anywhere are they. Or that they get shown porn?
I'm not sure OP if you sounded surprised that DS had seen porn on FB but (and I am not denigrating him, or you, or agreeing that this should be the norm or implying any negative to whether you were or weren't surprised) but I'd be surprised if he hasn't seen it - god knows many boys must be watching it if not at home on their laptop then using their smartphones to do so, and flashing the images around to their mates. I see this not with glee because the older I get the more I dislike every aspect of porn which is why I follow Fight the New Drug on FB which promotes love instead (how refreshing). I actually think our children are unprotected and been left hung out to dry in terms of child protection - the term used here in the UK is a joke. But I digress.
It also doesn't take account of the fact that they could decide, if you insist on access, that they will simply delete stuff that they don't want you to see so that when you conduct your review, that's nothing to see that rings any alarm bells.
The sad fact is that paedophiles have gotten away with abuse (if that's the main concern we're talking about?) for decades because they know that the child won't tell straight away (which is the thing we ask them to do of course) - but sadly history and case after case after case of abuse confirms that they don't do that. Paedophiles know kids won't tell straight away so all we can do is keep telling our children to tell us - and hope that they do (even though I too have these conversations with my children, I still don't think to myself 'well, you've ticked that one off the list Pollyanna' because there's absolutely no guarantee that they will tell me.
In any case many kids may have FB but it's not the main site many of them use - it's for 'older' people mostly, they're using other mediums to communicate and share pics.
I don't think it's right to be friends with them on FB and I don't think it's wrong to be friends with them on FB, it's one of a range of things you can do, which you have to choose which approach works best for you, but to believe that this has rendered them significantly freer from harm, is, I believe, actually a false feeling of security.