For goodness sakes--no one knows that this girl doesn't know she is adopted. OP, and her husband, are assuming that is why he was blocked when he attempted to contact them out of the blue on facebook when she was 13.
That's it. Unless OP is leaving a lot out, that is the only information they have to go on.
I read the information here thus:
1. 16 year old boy and 18 year old girl have an unplanned pregnancy. Girl and her family move away and boy does not make major efforts to see this child of his, hides the fact taht he has a child frmo his family and friends until he turns 18, etc. (so, not only does he make no real effort to see the child but in fact appears to make efforts to not be a part pof her life by hiding her existence).
2. Boy turns 18 and is asked to sign over his parental rights. He does so without finding out much information, without getting any kid of agreement to an open adoption, without talking to his parents or other adult for guidance, etc.
3. Afte signing over his rights, he tells his parents about the baby (why now? He is suddenly no longer afraid?) and htey try to have contact by sending gifts/letters which were retuned unopened. (could it be that the other family does not know his parenst were out of hvte loop and htinks they just want the joy of grandparenting now that all responsiblity is off the shoulders of their son?)
4. Young man does nothing for 11 years. Just lives his life as if he has no child. Does not contact her or the family and certainly does not send child support (not required since he signed away his rights, but if he were REALLY concerned for her wellfare that might be a good place to help).
5. at age 29 the man now decides he'd like contact so he finds the family on facebook and sends a message or tries to frined them (not clear which). We have no idea how this was worded or what he asked for, etc.
6. Family blocked him on facebook/ removed their facebook profiles.
7 Another 7 years go by in which the man does nothign about this daughter. Now she is 20 and he and his wife surmise taht perhaps she doesn't know about him or that she is adopted and convince themselves that they would be doing her a favour by exposing her to the truth if they contact her. Meanwhile, the man has not had any real contact with the adoptive family/bio mom in 18 years so he has absolutely no idea what this girl was ever told or not told about him or if she has any desire to meet him or not. He is just making wild guesses that support his/his wife's desires.