After kids leave their foster home, any future contact between a child and his or her former foster family is usually at the discretion of the parent-figures in the child's new home. Children can move on to another foster home, a kinship home (extended biological family), or back with their biological mother and/or father. (Read more about where and when kids move on to new homes.) This is one of many reasons why it's important to build positive relationships with the child's biological family and parents whenever possible.
Honestly one of my greatest fears has been discontinued contact after the children move. Saying goodbye and going home to an empty house will be hard enough, but the thought of saying goodbye and knowing that we'd likely never hear anything of these kids again is nearly unbearable. Fortunately, in our case, the parent-figure where the kids are moving has graciously said that we are welcome to visit with the kids "anytime."