A couple of easy ways to search for people on Facebook, without even having to log in. There's the Facebook People Search page which allows you to search for people by name, or to browse for your friends alphabetically by name.
Alternatively, you can try out the Facebook directory which lists people by name, and is rather slow.
Unfortunately, you then need to click on each name until you find the one that you want. Not too bad if you are looking for me - there's only 24 of us (though you might also need to try Philip Bradley as well!) but it's less than useful when you're looking for John Smith.
Alternatively, you could just go to your friendly search engine and search for "site:facebook.com their name" (without quotes) and then add in a few more keywords to try and narrow down the list to the person that you want.