I inadvertantly compared the two in recent months and found that Twitter was by far the more potent communications tool (in my unscientific test, at least).
What happened was, last November and December I changed the status on my Facebook profile to reflect the fact that I was looking for a job. My profile displayed that info for several weeks. In all that time I had one person approach me offering me some possible contract work. No more.
However, two or possibly three times since Christmas I have mentioned on Twitter that I am looking for a job and from that I have received 6-7 strong expressions of interest some of which are at the stage of swapping proposals.
The very first time Will Knott asked me why Twitter was so powerful was at the first Cork Open Coffee meeting back in March 07 and I remember telling him that the power of Twitter is in the network. Twitter continues to prove me right.