tinkly - I totally disagree with your simplistic approach to risk assessment.
Children placed for adoption undergo significant risk assessment in relation to
a) the risk posed by their birth family
b) the risk of their birth family "finding" them
These factors are all taken into significant account when placing children for adoption.
All adopters are warned about the new and real "danger" of social media and the internet for adoptive placements. Adopters will generally know the whereabouts of their child's birth parents and where they shop, what their usual movements are etc (this all forms part of the risk assessment on placement - and as to whether the placement is appropriate). They can therefore fairly easily avoid these places. It would also be highly unlikely for a child to be placed in an area where a birth parent might just "pop up" at a concert or similar.
What is FAR more difficult to control is the spread of information by social media and the internet as it is so much easier, quicker and not geographically (physically) limited. Pictures of adopted children, where there is a potential risk, are not put in the local media, or on any school publicity to minimise risk of the birth parents finding them and working out which school they are at etc. This is not only as there may be a risk of significant harm to the child from their parent, but also simply it is a matter of the adoptive family being free to live their life without the thought that the birth parent knows where they are and could "pop round" or pop into school at any time - it is normal peaceful family life with the freedom for the child to then explore their feelings about meeting their birth family at a time when they are settled and ready - when and if they wish to.
Where a member of the public unthinkingly puts a picture of this child on their facebook page it is far easier for a birth parent to stumble across it. It only takes a friend of a friend to comment on the picture, then it can appear on even more news feeds and it is actually extremely difficult to stop the spread of info. If you use facebook you will probably have come across the relatively common situation of seeing a friend comment on a friend's status and thinking "Oh I didn't know x knew y", or seeing your friend in a familiar place and deducing that they must have been visiting the area or similar...
That is why people in those circumstances are very careful to limit the exposure of their children to any more public arena. And flowergirlsmum is totally justified in accepting the instructions she has clearly been given about photos of her nephew without needing to fully understand the reasoning (or challenge it).
I am not arguing for the "no photos at all" side, but I do think that all parents have a duty to respect the wishes of other parents with regard to a photo of that parent's child. I would not put a pic of another child on any website or anything, other than if it is a close friend who I know is fine with it. One of my close friends doesn't like photos of her child on facebook at all - not my decision, I just edit her out if I'm putting a photo up. Tbh my DD is 10 and I now ask her permission before putting her photo anywhere, just manners IMO. So whether or not you agree with the the whole risk assessment and whether or not an adopted child could be put on facebook is irrelevant anyway IMO, if the parent doesn't want their child's photo out there you need to respect the decision.