It's not a boycott, just a dismissible nag message. Did you read the article?

Also, the number of people from these startups is approximately (data from quantcast):

4.1 million (jtv us) + 1.3 million (weebly us) + 1.7 million (reddit us) + 1.3 million (posterous) + 0.5 million (disqus us) = 8.9 million US people. Global reach is probably 2-3x that much, but quantcast doesn't offer global numbers for all of them (JTV has 26 million globally).

I'd say that counts as an established user base.

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.