Details are sketchy, but it appears that this will be a Web-based IM service that would allow Facebook users to chat with other people on their friends lists without needing to go through a third-party program. Additionally, TechCrunch's Michael Arrington detailed, the service would likely be based on the Jabber open-source platform, which would mean that third-party "universal IM" clients like Pidgin, Trillian, and Adium would be able to implement it.
Facebook representatives were not immediately available for comment.
It goes without saying that instant messaging is a logical step for a social network--it's an activity in which millions of Web users partake, and it would keep those coveted "user engagement" rates high. Facebook's obviously not the first one to have this idea: A number of third-party Facebook Platform applications facilitate instant messaging between Facebook users, and Arrington notes that those developer programs would be effectively killed if Facebook launched an in-house rival.
That said, other major social networks have some kind of in-house instant-messaging functions now: MySpace operates MySpaceIM, for example, and AOL's recent acquisition of Bebo will integrate the social network closely with its AIM client. If anything, it's surprising that Facebook didn't build something like this months ago.