Facebook is investigating an issue where some users do not receive an e-mail notification when someone sends them a message on the social networking site.

"We are aware that a small minority of our users are experiencing problems with email notifications," the company said in a statement. "We are actively investigating the issue at the moment and hope to resolve it soon."

Normally, Facebook will send the entire content of the message to a user's registered e-mail address along with a link that can be clicked. If a user is logged in, it will open up a Facebook page with the conversation thread.

The feature does have to be turned on, however. Users can turn the e-mail notifications on or off under "Account" then "Account Settings" and then under the heading "Sends me a message."

Facebook users have complained on a group called "Facebook for BlackBerry smartphones." Some users wrote that they had the feature turned on but still did not get messages.

The problem could reside with Facebook users. One person said she thought the feature was not working, but her e-mail application was sending the notification to a junk mail folder. The messages were delivered after being marked "not junk."

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Another user resolved the problem by marking the "facebookmail.com" domain as safe, which stopped the e-mails from being filtered by an e-mail application.

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