No, I agree that this problem has to be solved before this idea can hope to get wide adoption. Mr A posts something really evil on Mr B's facebook page. What happens next? Mr B files a complaint to facebook. Since this is a legal matter, the complaint gets escalated to the top. FB realises that this is out of their control and proceeds with a complaint to FBI. Now what if the data server with that comment is hosted outside the US? Right, the browsers will quickly ban this extension. However the owner of the extension can also change the default data server url in the config and the comment will be gone.

However I think that this hypothetical scenario will never happen because the idea will never reach this wide adoption. It'll inevitably be fragmented into a few segments with rules and moderation. Want comments from the theoretical physicist community? Subscribe to sci.data.org in your extension config. Want to know what 4chan folks think? Add unfiltered.comments.com to your config - it'll be an unreadable mess anyway and you'll need to make some non-trivial effort to add that url.

But I think there will be an archive with all the comments for research purposes: physically, it will be a big data server that keeps connections with all other data servers and pulls all the comments it can.