Zero percent, and report for spam.
> Date: Sun, 17 Jan 2010 22:13:33 -0800
> Subject: [twitter-dev] @ Message read rate for non-followers
> From: abstar...@gmail.com
> To: firstname.lastname@example.org
> Hey Guys,
> Do you know what % of people read @ messages if you are not a follower
> + targeting them based on keywords or search api's?
Windows Live: Keep your friends up to date with what you do online.