MLS vs Realtor.com or Trulia?

[Mike Abramowitz]

I was hoping someone would be able to help me out with how much of a difference the MLS vs something like Realtor.com or Trulia is as far as searching for properties. Obviously MLS is the best way to go about it. But do the majority of houses make it to sites like these? Is there a reason that most do/don't make it?