According to a post on Webpronnews:
“Messages May Be Filtered Out – While a Facebook.com email address is offered, it’s not required. Whatever the source of email, Facebook promises to use “Friends”, “Likes” and other clues to filter email. So a fan who signed up for a band’s email updates, could find their emails automatically shuttled into an “other” folder if they have not also friended the artist on Facebook.
Facebook says that their system will get smarter over time, so that people that aren’t friends on Facebook but communicate often will show up in the stream. How repeated one way communications like email newsletters will be filtered or allowed to pass into the users main stream is unclear.”
You can read more on the above post here.
While the above post relates to musicians sending e-newsletters, I think all email newsletter publishers should monitor the development of Facebook’s messaging system.