I'm going to disagree that the BPs have the legal right to do what they wish with those photographs. I suspect it's never been challenged, but I suspect legally that actually, as legally BPs are now strangers (legally), and APs are the sole people with PR, and are the only parents (legally), that BPs have no rights to publish images that are of unrelated (legally) children.
However, I would think the risks of communicating with facebook (and that they would likely ask for proof of PR), and then them passing that on to BPs may mean it isn't worth it. However, facebook should communicate the wishes of those with PR to have images removed without breaching AP's data.
I would be concerned that it would provoke BPs to potentially more difficult behaviour, off regulated SM, for example websites etc.
I would also stop contact immediately if any contact content was shared.
I would also change my children's names, if they were publishing them.
Our children have a right to privacy. And whether the BPs contested the adoption, or didn't, whatever the reason for removal, the children come first.
I think you could report to facebook, with the risk BPs would then up their behaviour, and do you want that risk? Presumably they're old photos? Only you can weigh up the risks. I would ask SS advice, but then weigh that up, as SS don't have to pick up the pieces on this. It's so difficult.