Adoption and פרו ורבו

Tamar Novetsky writes:

If you have children and immediately give them up for adoption, have you done the mitzvah of puru urevu? 

A couple of points about פרו ורבו before I try to answer that question.  First, remember (as you learned last year) that it is a מצוה from which women are exempt, so we're only talking about the father.  The topic that is more often discussed is whether you fulfill the mitzvah with adoption.  There are opinions that you do, based on the maamar chazal that "hamegadel adam betoch beyso kieylo yilado" that someone who raises another child in his home is considered as if he fathered him.  I assume that this is not to the exclusion of the biological father, but in addition to him.  I think that once you have children (a boy and a girl in accordance with the opinion of Beis Hillel), you are yotzei whether or not you actually raise them.