LiveJournal (which was something like the MySpace of the 1990s, only not owned by the forces of evil and not spammy with ads) is
testing its own instant messaging service. The service is technically a Jabber/XMPP server connected to LiveJournal, and using authentication and social-network data from LiveJournal's database. And, being XMPP, it can communicate with other open XMPP networks, such as Google Talk.
-- your Jabber Roster ("buddy list") is integrated with your LJ friends list. If you friend bob and bob friends you, both of you can see each other online. It has to be mutual. Friends that haven't friended you back show up as "pending subscription" in your jabber client, kinda grayed out, depending on the client.
-- if you add a LJ person in Jabber, it won't automatically friend them on the site, but next time you use the site, it'll prompt if you want to. It's imaginable there's people you want to chat with, but not befriend. Our strategy is "least surprises".
According to the comments, once the server works, there are plans to add an AJAX messaging interface, sort of like GMail's Chat mode.