Seesmic says Seesmic Desktop is going to replace twhirl. They just released an update...

Seesmic Desktop 02-rc2

...that adds Facebook account integration. The smaller font used by this update makes it look a bit neater. But, I think Seesmic Desktop takes up way too much screen space (compared to twhirl). And, I don't like having to switch accounts for the text entry box. It would be better to have separate text entry boxes for each account to avoid confusion.

I really hope Seesmic keeps twhirl around and continues to develop it. Seesmic Desktop may appeal to some people, but not me.