I tend to find that Facebook is wonderful if you post articles on the latest or controversial topics, it gets people talking and sharing their views, which in turn retains interested viewers and creates a domino chain of discussion from there. I have seen this happen all too much. Every time a discussion spikes this way, I see an anywhere between 50%-200% spike in traffic to my article sites.
Twitter was engineered for short comments that trend. This is constantly evidenced by my success in promoting Clickbank offers with tempting, short, and sweet messages describing the benefits of the product in question. It doesn't work so well for sites or individual articles.
In the end, I prefer them both for different purposes. With one, a specific venture fails while with the other it sees a booming success, and vice versa. They are both tools, and knowing how to use each one to its utmost potential with the platform given makes a huge difference in whether you fail or succeed.