It looks like Facebook provides some very weak, but better than nothing, webcal support, that can be used to have your Facebook events show up in Evolution, and thus, on your Ubuntu desktop calendar.
- Log in to Facebook and then browse to http://www.facebook.com/?sk=events
- At the bottom, find the links that says, "Export Events"
- Right-click the webcal link and copy the link address.
- In Evolution, go to the Calendar section.
- Click on New->Calendar.
- Under Type, choose On The Web.
- Paste the link you copied from Facebook into the URL field.
- Give the calendar a name, like "Facebook", and put your Facebook username in the Username field.
- Click OK and then supply your Facebook password when prompted.
This webcal is read only, so you cannot add Facebook events through Evolution, and invites won't be synched back to Facebook either.