There are a number of different ways to find old friends, acquaintences, or family by using the internet. I don't like most of them for one reason or another -- money, ads, whatnot.
Usually you must have the person's name, which I don't always know, or maybe I just know their nickname. I know people who have lost track of family member, or want to find their birth parents.
I recently ran across a great (and free) service, that seems to cover all the bases. If you "walk the earth", this comes highly recommended.