Despite what a lot of idiots have to say about what s4s means, it normally comes into play with myspace sites (i.e. layout/contest/html sites).
Most sites have a support code
, and when asked to s4s, two sites swap codes and paste into bulletins so their friends can see the code, and hopefully they get added.
It's not just for "myspace nerds" or whatever. Some sites out there have actual meaning.