Allow me add some perspective and number to this discussion.
I have a fan page for my website on Facebook that has over one million fans (nevermind how I got them--I got lucky); however, whenever I post a status update with a link to my website I can expect about 1000-5000 visitors.
Recently FB has started displaying user statistics to page admins, so I can now see that out of those million fans, only a little over 300k are active on a monthly basis (FB doesn't define what active means in this case, but my assumption is that they interact with my fan page at least once per month).
So as you can see, even with massive numbers FB is not a good traffic generator. There are a variety of reasons that only about 1000-5000 visitors trickle through with each linked status update; however, I won't go into that in this post.
Let's move onto Twitter. As many have said, Twitter consists of some good 'content', but is mostly junk. For example, a lot of Digg users utilize Twitter to promote their submissions to digg (for those who don't know, digg is a social bookmarking site). These types of updates, as well as other promotional crap is all over Twitter. This does not mean, however, that Twitter cannot generate traffic. If you are willing to spend the time, and create a following on Twitter, the CTR from the follower base is much better than FB.