depends on what kind of info you have. If you have last address, first last name or more info then you can use one of those search systems. I have known people to use them, they do work, but they cost money.
Google also can find some stuff as well.
Also if you think they are involved with the internet at all check out myspace, facebook (alot more likely if they went to college), and friendster. Its insane who all you can find on those place. Look through school alumni and such. Also there is classmates.com
If willing to spend a little dough, not hard at all. If wanting to find them for free, might take a little more hunt work.