Sites also don't have to be 'important' for them to matter to people.
Of course, it's all ridiculous (for now). Most people in the real world have better things to do than troll around on Q&A sites hoping for karma. Until someone figures out how to convert karma into a competitive paycheck, this is all nonsense.
Someone can be clever and knowledgeable, and still be a poor employee or co-founder. (And, my least productive times are often when I'm most interested in what's happening on reddit/HN/etc.)
What is good about sites like StackExchange is that they encourage public discussion, and knowledge sharing. So if you have a StackExchange account, an HR rep doesn't have to just take your word for it that you're a well spoken, helpful individual---they can peek over on StackExchange and actually see your answers, comments, and questions. The karma attached to them is a meaningless indicator of popularity and not quality.