Another perspective: people wear hats. We cater the hat to the audience we're addressing. This isn't just a web phenomenon - we've crafted our identities in the real world for thousands of years in order to manipulate the target audience in some fashion or another. For that reason, it's not a smart idea to link a single human to a unified web presence.
Facebook login makes sense in its convenience, but in reality, it grates against something that's pretty important to a lot of people: how they present themselves in front of an audience.
