The service (codenamed "Project Titan") aims to give users more control over their inbox by allowing customized viewing priorities and visible conversation histories.
Users will also be assigned an "@facebook.com" email address, but don't call this an "email killer," says Facebook CEO Mark Zuckerberg.
Although the service is invite-only (for now), users are already reacting to Facebook's announcement. See how 16 Twitterers responded to Facebook Messages, then share your thoughts with us in the comments below. Read on for a detailed overview of Facebook's new product.