I recently got the G1 and love it, but I was disappointed to find that there wasn't an official facebook app out yet. Although an official facebook app is rumoured to be in the works, there is nothing out yet at the time of writing. The next-best thing is Babble, which includes pushed facebook notifications and most of the features you'd expect from an app. Here is the QR barcode to get the app on your G1:

Babble Lite for Facebook

Babble's Website