Ha. Sadly, I know this, because I used to have a roommate who went through Facebook fasts. If it exists- You can still see their picture/name- it's like searching through a phone book. Pretty sure you can still send them a message. And you can see things they've been tagged in if you have friends in common. You just can't access their profile. (There are ways around this. I know several people who use some sort of variation of their real name to prevent random people from finding them.)
And I know you can delete- people who role play do so all the time. And some of the people I knew who were education majors deleted because professors really emphasized the fact that social media could be a liability.
So basically, the only way they could completely avoid him is if they changed their screen name, changed their profile picture to...I don't know...Mickey mouse, made everything strictly private, and told any acquaintance they might have in common not to tag any photo or link them in any way, he could still find them on Facebook. Kind of freaky. I actually have known few people who realize just how much people see of their Facebook "breadcrumbs trail". And the other thing- the kid would have to know about at least some of this because they couldn't shield her totally without her cooperation. If she's twenty, she'd be a rare kid indeed if she didn't utilize about 10 different social media platforms. That must have occured to her adoptive parents. If not them, her biomom. My parents have been trying to get my youngest sibling to put more privacy settings on but short of hacking her account, they can't force her.
That said, the simpler solution for all of that? Would be to either ignore his message completely or send a terse "she's fine and never wants to meet you". I think that message really triggered a fear for them. Either it was from the gut, overprotective (makes sense) or there's more to the story that we don't know. We really have no idea how the guy acted towards their daughter or her child or even them over the course of those two years. Even the OP doesn't know that. We don't know if the kid even still lives at home. And I still don't understand why the biomom does not have a bigger role in this saga- because the guy was presumable closer to her than to her parents and there's no indication that she's estranged from her daughter/adoptive sibling or her parents.
I don't know, I just feel like there's something off about the whole situation. But it could just be lingering emotional trauma, perhaps. Teen pregnancy and adoption are just really heavy issues more often than not....