I like the concept behind Facebook--it's a great way to stay in contact with large groups of friends, and it's a handy way to find out what people are doing. However, I doubt I'll ever post very much to Facebook; instead, I'll continue to use my blog as my primary means of expression.
The main reason is this, found in Facebook's terms of service:
The following sections will survive any termination of your use of the Facebook Service: Prohibited Conduct, User Content, Your Privacy Practices, Gift Credits, Ownership; Proprietary Rights, Licenses, Submissions, User Disputes; Complaints, Indemnity, General Disclaimers, Limitation on Liability, Termination and Changes to the Facebook Service, Arbitration, Governing Law; Venue and Jurisdiction and Other.
So whatever you post to Facebook, they own forever, even if you decide to cancel your account there. Bear that in mind before you decide to share your next great creation with all your Facebook friends!