The application is intended as a "one stop" for Twitter, Facebook, Google Reader, Flickr, Digg, and any site that supports RSS.
I've used Eventbox for some time now, as well as Socialite betas, and I was impressed enough to buy a license before RealMacSoftware purchased the application and changed the name. So far, I am impressed.
Twitter support includes multiple accounts, and it makes it very easy to send Direct Messages as well as @replies. Socialite also includes support for Twitter's new "Lists" and "Re-tweet" features. As far as I know, Socialite is currently the only native Mac Twitter client that supports lists. The application is the only way that I ever bother to keep up with Facebook, and it can do so without all the "cruft" (ads, for instance) in the browser interface for Facebook. Socialite is also a great way to keep up with your Flickr contacts.
Socialite sells for US$20.00, and a demo is available. If you previously purchased an Eventbox license, you will be contacted with instructions on getting a Socialite license within 24 hours. Without a license, Socialite is limited to three services, which will probably be plenty for some people.
Download the 1.0 and check it out for yourself!
Update: RealMacSoftware's website seems to be taking a dirt nap. You can download the 1.0 version directly from Amazon's S3 servers here. (MD5SUM = f06c92ce51b7f9a3db96d5e9051adbdb)