For example, I know Frank means that I can tell you something as to what Frank is doing for a living and it is possible I met him a couple of times.
I know about Frank means I am not in a position to tell you anything as to him.
Probably I heard from some other person as to Frank. Simply speaking I know he exists. No details.
So how do you say in French 'I know about him'.
The thing about "know about" is it's very vague. It can imply things like: On m'a dit ce qui s'est passé avec Frank, Je suis au courrant du problème de Frank etc-- I think almost inevitably, you end up saying something more specific in French depending on the exact situation. And indeed, j'ai entendu parler de Frank is one of the possible interpretations.
Sorry to break in but I have been wondering about this particular construction for a while. If asked, "Tu as entendu parler de Sarkozy?", why are the first two following responses correct and not the third.
Oui, j'en ai entendu parler
Oui, j'ai entendu parler de lui
Oui, je lui ai entendu parler.
I think this is me just confusing stressed vs indirect object pronouns but I can't quite put this together.