Pre-Adoptive Placement - how does it work?

From what I hear many areas like to hang on to their Foster families so they would rather place the child with a foster/Adopt or Adoption only family. But like the PP stated. It also depends on what they child needs. If the child needs a two parent home to thrive then they will not place the child with a single parent. It also depends on who is willing to take certain special needs and not.