I was discussing politics this weekend with a friend/coworker and we got to discussing McCarthy. My friend asserted that years after the fact it was proven that there were actually quite a few Soviet spies or informants in the US, and that McCarthy was at least sort of right on that front.

I honestly could believe this, but I'm a bit more skeptical than my friend. I've looked into it a bit, and learned of the Venona Papers which implicated the Rosenbergs. Is there any other "hard" evidence of the scale of Soviet espionage in the US during the red scare? Or anything to point me in a fruitful direction?