Because you might want to leverage the reputation, credibility and following you built in one online community to another. Example: I learned about patio11 here on HN and was impressed by him actions in this community which lead me to follow patio11 on Twitter.
I agree that one of the great things about the internet is that you don't have to have the same identity in different communities. It's optional. Facebook's business model gives them a strong incentive to remove that optionality.
I agree that one of the great things about the internet is that you don't have to have the same identity in different communities. It's optional. Facebook's business model gives them a strong incentive to remove that optionality.