I wonder if enough of the core-base was eroding that they were forced to make this change. I've been logged out of Facebook since October, and I know a few other friends who have as well. That's not really relevant, except that this group all had membership on Facebook at the 1 year mark - early 2005.
Nobody is posting or reading stuff in public... Except older mostly female people (i.e. the "my aunt" stereotype).