You can mark your account as private on Twitter, if you like.

But you can't do the opposite... I believe this is the point the GP was making.

Sure you can. Set status permission to "Everyone". Facebook introduced the feature here:

http://blog.facebook.com/blog.php?post=98499677130

But on Twitter, that seems like a step towards outright account deletion.

None of my twitter contacts are on my FB.

Facebook allows public verbal diarrhea too now