Most people on SLS want to meet and screw, and aren't too interested in forming friendships unless it just happens. Not their primary goal. That being said... I've made some awesome friends from people I've met on SLS so it isn't impossible.
OkCupid or other poly-friendly dating sites would probably be more useful if you're looking for a loving relationship over just a sexual thing. Even a FWB without the love aspect that poly generally entails would probably be easier to find within the group that is looking for companionship instead of the group that is looking for sex.