Mac-using Facebook fans are going to be happy to learn about Facebook's experimental Desktop Notifications app. Desktop Notifications sits in the menu bar, and pops up notifications (using Growl if you have it) when they occur. It also gives you quick access to your news feed, profile page, and quick ways to update your status or start a new Facebook email message.
There are two things about Desktop Notifications that are compelling, yet have nothing to do with the app itself. The first is the fact that it's a native Mac application, rather than yet another Facebook client written on Adobe Air. The second is that it was actually
developed by Facebook themselves [Update] Thanks to Raul and Nate for pointing out in the comments that the application is marked as "not developed by Facebook", though one of the developers listed works for Facebook. It's unclear at this point how serious this project is; it's clearly marked as experimental, which is clearly becoming the post-Gmail way of denoting that something is beta.
In terms of raw functionality, Desktop Notifications is pretty barebones, since most of what it does is take you to a particular Facebook page. Personally I kind of like it that way. It's relatively light in terms of memory usage, and uses virtually no CPU cycles at all unless you are actively interacting with it, which is exactly what I want from a utility that is running all the time.
My one beef is that the built-in hotkey that pops up a status update dialog box conflicts with another utility on my system, and there is no way to modify it. Since Desktop Notifications is still in the experimental stage, that's a pretty small complaint.
[Update] Commenter Scott points out that there is a preference setting to change the hotkey.