Perhaps I'm misreading your analysis but it seems to me you're suggesting that the US visitor count for each of those sites is 8.9 million people total? I would suggest it's actually much lower - the chances are that the people who use reddit are the same sorts of people that use posterous and disqus (and so on). You're potentially counting 1 person who uses all 5 sites as 5 separate people.

The problem I see is that - particularly for reddit, posterous and disqus - the audience is mostly technical (or at least fairly Internet literate). These are not the types of people who need to warned about IE6.