Facebook just launched their previously-rumored IM functionality. It offers the opportunity to chat with your facebook friends in real-time over the network in your browser.
Kottke mentioned in the past that Facebook is the new AOL. Over the last few years, they've taken increasingly greater steps towards the walled garden approach: sure, interact with our users, but only if you do it via our proprietary methods. AOL neglected HTML in favor of RAINMAN. For Facebook, you have to dive into a world of FBML and FQL. It's a tradeoff between consistency and flexibility, to be sure, but the discussion of the viability of a closed system like Facebook applications on an open system like the internet is rather substantial and best left to a different blog post entirely.
After an interesting few years of back-and-forth battling, AOL eventually ditched their walled-garden approach for their IM protocol and moved towards some openness by publishing guidelines to OSCAR for 3rd party projects like Pidgin or Trillian to use. It could be argued that the growth and ubiquity of the AIM network is sustained through the ability to run AIM over Trillian, Adium, iChat, and others. My entire generation (say, those in college right now) tends to use AIM nearly exclusively, with few of my personal friends using the official AIM client itself.
The thing that's interesting about today's development is that Facebook has diverged from AOL's model, albeit in a backwards way. Facebook Chat uses their existing web interface to deliver their new functionality. Whereas AOL moved from walled garden to tentative openness, Facebook moved from walled garden to, well, walled garden. You can only chat while on facebook.com, you can't use it via their Facebook iPhone/mobile apps, and you can't use a third-party app to interface with the network.
This is a relatively new thing, at least for me. I've dabbled on most major networks: AIM, MSN, Yahoo!, Jabber, and Bonjour. The common link between them all is that they're all based on relatively open protocols, at least to the point where you can use them in an Adium or a Trillian. If there was a closed IM network, they at least provided desktop functionality. Facebook doesn't. A straight web-only interface makes it difficult to really be a viable communication medium.
There are a few reasons why I enjoy Adium (though my arguments will likely apply to Pidgin or iChat or whatever your flavor of IM client). For one, I'm a habitual conversation history tracker. I have logs dating back six or seven years. It's helpful to me—I can't tell you how many times someone says something, I close the conversation and immediately forget which time I was supposed to meet them or the line of code I was supposed to check out. Facebook logs your conversations between sessions but not permanently, supposedly (Facebook, as its customary style, is fairly ominously-vague when it comes to your personal information and longevity).
Secondly, you lose out on the natural behavior of your operating system. I actually just received an IM from a friend right now. How did I know? I'm in a different Space in OS X, so my IM windows aren't visible. It's because of OS behaviors that, as a user, you grow accustomed to. Step one: I hear my normal IM ding. Step two: I get my Adium duck flapping its wings on my Dock, with a OS X-standard "1" icon superimposed on the Adium icon, signaling to me that I have one message to check out. Step three: I get a growl notification that tells me the sender and their message, if I want to bother to check that section of my screen before command+tabbing over to Adium to actually read or respond. Beyond that, there's a multitude of OS-specific behaviors you expect: on a Mac, I expect command+W to kill the current conversation tab, shift+command+]/shift+command+[ to tab between conversations, and hey, Adium does a great job with interacting with Address Book to grab user icons and first and last names. You don't get any of this with Facebook Chat.
Even the web interface isn't very standard. For one you have to remain on facebook.com to receive messages, you have to resist a stale session (after 10 minutes or so you're auto-logged out unless you visit another page), and some pages don't even work (try the "People You May Know") link on the main page.
With these limitations, it might not surprise you to learn that I'm not too enthused about Facebook Chat's prospects. I can't really see it being much more than a "hey, I'm in a cluster on campus and can't get on AIM or MSN... here's the info you wanted". I don't necessarily see it as taking the place of existing networks, particularly because it's so closed. Not only is it web-only—you can't use it with Adium—but it's also Facebook-only—you can only use it with your Facebook friends. That kills prospects of those who want their IM handle public and don't want to have to befriend every user.
Google ran into similar circumstances with GTalk: they gave easy web access to GMail users to talk to existing email contacts, but at the same time pushed open and accessible Jabber standards to allow interoperability. If Facebook wants to really push Facebook Chat (which, since they tossed a floating div on the bottom of every one of their pages, seems to suggest this), they would be smart to take a move out of Google's handbook and add some open operability to their network.