On facebook, you do the usual stuff that you would do on any social networking site: set up your profile page, a profile that projects your professional personality. Look at other journalists on Facebook and see what works.

Tip: Think about what the first 2 lines of your 'about me' should look like, for this will also come up in any search result, including Google.

Stuff that matters:

Make many friends in your professional and others.

Use the status tool for maximum benefit. It works like Twitter and use it to broadcast any interesting new thing you have just done, thought or found online.

Facebook and data ownership: This is also true with all social networking sites and web 2.0 sites in general. Always check out the service’s Terms of Service (TOS) before joining or posting something you have worked hard on. Check for whether they will let go of license to using your (yes, many free web 2.0 hold the right to do so) once you have stopped using the service. More here.

Two great Facebook for Journalists guides are here:
Shane Richmond of the Telegraph of London on Facebook for Journalists
Save the Media Blog on Facebook for Journalists