Facebook isn't trying to reinvent the web, but rather take what already exists and make it better. The goal of their new messaging system is to unify the many modes of communication you already use so you don't have to jump between applications and devices to manage the different ways you and your friends like to communicate. Furthermore, Facebook wants to use what it already knows about you to help you sort through your messages, allowing you to spend more time dealing with the messages that are important to you and less time wading through emails, texts, and IMs that you don't care about.
One Centralized Messaging Hub
Facebook's aim is to centralize your text-based communication into a single location, much like Google Voice has done for voice communication. It doesn't really matter what device you're using or how you want to communicate because Facebook will continue the conversation through the most applicable channel. You can switch computers and devices or modes of communication and Facebook will adjust right along with you.
If you're skeptical of the usefulness of a centralized messaging hub, you don't need to look much further than your smartphone to see how well it can work. Smartphones are great communication tools because they're capable of combining all different types of communication (email, SMS, IM, voice calls/voicemail) into one easy-access location. Facebook's new messaging service is attempting to lift this paradigm but bring it not just to your smartphone but your desktop or laptop computer as well.
Speedier CommunicationFacebook's goal in developing a more comprehensive messaging system is to improve the speed of communication between people. It's not that email, itself, is necessarily slow but that when you send a message you have to fill out a lot of information. In addition to the message you send, you're supposed to supply a subject, need to decide between To, CC, and BCC, and make other little choices that add a little more formality and cognitive sludge that makes communicating slower. In his presentation of Facebook's new messaging service, Zuckerberg claimed that using Facebook messages or SMS is the preferred, faster way of communicating among young people. While email certainly still serves its purpose, when your goal is simply to have a conversation it can provide a larger set of features than you really need. Facebook aims to simplify your communication, removing any hurdles along the way to the send button, so your communication feels more like you're speaking to someone and less like you're writing a letter.
Highly Effective Message FilteringGmail's Priority Inbox uses your communication habits to judge which messages are more important than others and sorts those messages for you. Facebook attempts to do something similar, but it instead uses what it knows about your relationships with people. If you're honest with Facebook about who you're actually friends with and set your privacy settings to enable communication with the people you want, Facebook will provide you with a social inbox that contains emails, texts, and instant messages with people you (presumably) want to communicate with. Facebook will also provide an inbox for messages with people who don't appear on your friends list (or are friends of your friends, depending on your privacy settings)
It Works if You Let It
As mentioned before, Facebook uses what it knows about you to make your communication more effective. If you friend a bunch of people you don't know, it definitely work work as well. If you want to make use of this simple but advanced communication system you have to work with it and not against it. It's easy to criticize how useless it can be if you don't use Facebook as it was intended. The idea behind Facebook is to better communicate with your friends and not several hundred people you don't know. If you're out friending just about everyone you can find, it might be time for a friend purge. (With the National Unfriend Day coming this Wednesday, you have an official excuse.) While you should be able to use Facebook the way you want to use Facebook you can. If you want Facebook to be an effective tool for communication, however, it will be only so much as you let it.