Thank you TA McCann and Brad Feld for being on that very short list and helping me experiment with Facebook. There has been a bit of on-line debate about Facebook vs. LinkedIn so thought I would add my two cents.
I think this comes down to personal utility.
LinkedIn provides me utility because it is a dynamic but passive way to stay connected AND because my professional/personal contacts utilize the service. I'm not sure I would pay for LinkedIn although the ability to keep track of the folks outside my 15 or so most active contacts is appealing and updates are driven by the contacts themselves not by me (versus me managing Outlook Contacts). In order to be included in my network, I need to know you, have met you, or at least have something in common (Arthur Andersen alum, etc.). To me it is a "I know where to find you if I need you" solution and a way to keep tabs on folks as they move from project to project.
Facebook is also dynamic but requires active participation AND my professional/personal contacts DO NOT utilize the service. Admittedly, I have not really attempted to develop more connections on Facebook and signed up more out of curiosity than anything else. The same can be said for participation in MySpace, Orkut, etc. where I may be a registered user but I do not actively use the service. Besides, a system that asks your relationship status before anything else is not really where I am in my life. Could Facebook do what LinkedIn does for me? Yes. Is there a compelling reason to change? No, but I am open-minded and curious if its personal utility will increase as adoption continues.
Here's a couple good posts from Mike Feinstein including this one on being a social networking stowaway which outlines several services and his use of them. Also check this one on business social networking.
On a side note, I am still tinkering with Twitter and am finding it a reasonable way to update my whereabouts for the day rather than sending out a blast email. Surely there are other use cases for it but broadcasting my location/availability is one that fits my current needs.