From Facebook Developer Wiki

Description

We will be transitioning from requests to encourage users to share via the Inbox, Invites or the stream. Requests will be removed approximately 30 days after we launch our revised Share dialogs, which are focused on user intent.

Estimated Timing

Late 2009 / Early 2010

Details

Going forward, the best way for users to send messages to each other from within an application will be via our improved Share dialogs. Users will still be able to intentionally share stories into the stream to tell all of their friends about content they create and discover, and will still be able to invite their friends to check out applications.