This extension should go through and (try to) mark all of your recent posts as Private (visible to 'only me').
1. Navigate to your Facebook page. NOT your homepage, but your PROFILE page. For example: http://facebook.com/your_user_name
2. Click the red Facebook Privatizer button.
3. Wait. If it stops for more than about 10 seconds, click the button again. This may take a long time.
This is a BETA.
Facebook makes it REALLY HARD to do anything like this, because they want your data to remain public.
It may skip things or go very slowly. It may also stop at some point, at which point you can just press the button again to get it started.
If you find this annoying, please feel free to inform Facebook that you'd like for them to bring back the ability to hide your old posts in bulk, at will.