Should adults be telling any tween or teen how "hot" they are...probably not. I am 31 and if a teen were to message me on Facebook or try to friend me I would ignore them (unless they were one of my friend's kids).
However, there are creeps out there and the responsibility is to teach your child how to be smart and safe online. Back in the day kids were taught about not taking candy from strangers or that if they are approached and don't feel comfortable to scream until another adult intervenes. It is not all that different online. If your child is prepared for the real dangers online they will have the tools to react accordingly. It isn't easy but very little in parenting is.
I would be much more concerned with my teenage daughter telling a 25 year old they look hot than the other way around. She can ignore him, initiating the contact means the choice is taken out of her hands and put into his.
It astonishes me how parents know so little about what happens online. I know of people with tween or teen kids who don't know anything about social media. If you don't educate yourself on the medium you can't prepare your kids for it. I am in Information Technology and follow a lot of blogs and podcasts that bring up what you are talking about. Education is the best form of prevention. It is a cliche but it is true.