Mum in my son's year whom I'm Facebook friends with has posted a gushing congratulations message to her DS on his GCSE results, details of said results and how proud she is of him etc etc.
The DS lives in the same house. She can literally just speak to him and tell him how proud she is, as I've done with mine. I can't therefore see any reason for posting to him on Facebook, other than it's a stealth brag.
I get that some people have hoards of RL friends/relatives all over the world etc and FB is a then a good way to let them all know family news, but she isn't in that situation.
She does tend to do this for every single thing the DS does well, and
I fully accept I may just be old and out of touch with how people use FB as I'm not one to splash every bit of my life all over it. I just don't get why she address congratulations to the DS? Just tell him to his face 