Okay I got things working. It works for me with Chrome and Firefox on Linux.
Setup:
1) get your yubikey
2) got to clavid.com. use the yubikey registration. choose a username which matches your yubi username to keep you sane.
3) your are not yet fully registered. you need to login to clavid.com and complete registration by setting up your email address. it will send you an email with a link to activate your account
4) login to clavid (Secure Login link on right which changes to be Reserved Area link if you are logged in which is bad user interface design)
5) within the clavid site once logged in go to the Reserved Area and click Account Settings and then Your OpenID Sites. Add a new site. NOTE you need to know the exact url you are going to use to login to facebook so type it carefully "http://www.facebook.com"
6) go to facebook. if you at this point you should logout of facebook ("Account > Log Out" top right). Unselect the box "Keep Me Logged In" and login with your username and password. this step of logging out and logging in is simply to clear the Keep Me Logged In option from the facebook homepage as it seems to interfer with the automatic login once it is setup as follows.
7) go to Account > Account Settings. On the Setting tab halfway down the page is Linked Accounts. Go into that.
8) Add a new linked account of type OpenID... and enter the url as your username dot clavid dot com "http://username.clavid.com" where you use your username as part of the url.
Now to test it:
1) close down all browsers to ensure you are fully logged out.
2) go to http://username.clavid.com (once again use your username in that - you may as well bookmark that page)
3) click Secure Login on the right and select Yubikey login. Annoyingly you have to retype your username.
4) now go to http://www.facebook.com (IMPORTANT: THIS MUST BE THE EXACT URL THAT YOU SETUP AT SETUP STEP 5)
5) you will see the facebook login screen but the browser should still act like it has not finished loading the page whilst it does the autologin. This take 10 seconds on my machine so be patient.
6) your homepage will automatically load
All in all it is a real hassle to set this up. I would not even try to attempt to persuade someone who did not work in IT to attempt to get all of this working. So right now I would say it is for early adopters.
p.s. another gripe with clavid is that it says it has the worlds easiest yubikey registration but it does not ask you for your email on that to send you your registration link automatically.
p.p.s this does not work for me using the epiphany browser on linux. since i don't use that browser much at all i am not going to see if fiddling with the cookie security settings would get it to work.