The study compared data from 1985 to 2004 and found which the mean number of people with whom Americans can discuss matters important to them dropped by nearly one-third.

句子红色的which为错误，应改为that（引导宾语从句）。

划线部分为定语从句，修饰people。whom指代people。with whom介词短语在从句中作状语，修饰谓语动词discuss。

to them修饰important，构成形容词短语，作matters的后置定语，them指代Americans。