This is actually a really interesting point. However, its clear that absolutely any method of communication is vulnerable to the very same "social DOS attack," especially if we're imagining it to be perpetrated by the government.
Personally, I've spent quiet some time on the .onion network and never been ambushed by child porn. Don't you think your problem is solved on the .onion network the same way it is on the clearnet: with moderation on a website-by-website basis?
I certainly see links to it fairly prominently displayed on the Tor onion network, which I know would drive off at least 75% of users.
What I'm saying here is that there's an interesting unsolved problem and that this problem might be the thing that's blocking the adoption of these technologies.
It's also a critical mass problem. I don't think you could run that sort of attack against the Internet because it has over a billion users. Once the network reaches a certain mass, it becomes far less of a problem. The problem is that CP-wankers (and possibly attackers) instantly colonize darknets, rendering them quickly polluted before they have a chance to escape their nascent phase. Like I said in another post: you could apply game theory here.