I notice that CCleaner has the ability to exclude specific registry keys from the search, but not to include them.
I would like the ability to include specific registry keys to delete.
Alternatively, you could add Steam's friend cache (HKCU\Software\Valve\Steam\Users\*\Friends) to the Applications tab.
Currently the friend cache logs every single person you ever play a game with, and never purges it -- Valve is aware of this oversight and is trying to correct it, but in the meantime the only way to clear out the unwanted cache is by manually deleting the registry key.
More details here