@tjgammon,
Is it just that one person's activity is hidden, or are all of them hidden?
For one, then it's likely their own privacy settings. For more than one, there are some changes that have occurred. This might be one of them. Understand that I'm not telling you anything definitive because FB is the biggest website tester on the planet. They are constantly tweaking and changing things. This is why you might have settings that your friends do not, even if everything appears to be identical. It is the company testing things out.
You should, though, be able to go your Timeline review (just go to your own wall and you should see this) and see what you have done. Check your own privacy and notification settings while you're at it as you may have altered something or other.
Stumble It!
Tweet This
Bookmark on Delicious
Share on Facebook
Share on MySpace