Security by obscurity and personality shards
Adam Rifkin on TechCrunch has an interesting article about Tumblr and how it is actually used.
The thesis of the article is that Tumblr is used more openly and for more sensitive things than Facebook because the privacy model is so much easier to understand and implement.
If you have five interests and corresponding social circles, just set up five pseudonymous Tumblrs. Each then becomes its own independent social space with minimal risk of cross contamination.
While all of those Tumblrs are public and discoverable, in practice they are not easy to find and unlikely to be stumbled upon by undesired individuals. This is classic security by obscurity.
By contrast, Facebook wants you to put everything in one place, then use various settings to try to ensure that only the desired subset of friends, friends of friends, or the general public have access to it.
This ties to the case I have been making for a while that people want to be able to separate their various personality shards among their various social circles. Even with access controls, using the same account for all of them may be too much connection and the odds of accidentally releasing information to the wrong people is too likely.
I would like to see something like Tumblr provide stronger abilities to restrict discoverability, but it represents an interesting and growing alternative model to Facebook.