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This is a sample largely consisting of web developers, and people who know that you can have more than one account. That conclusion doesn't follow, in my opinion.

I think you mean "That could mean that FB..."

There's less than a thousand respondents in this HN poll, and HN isn't likely a representative sample of FB users.

HN is a sampling of about 0.01% the size of Facebook. There's absolutely no way you can make the argument that they have only 20% the users they claim.