Still, we have this, from the article you referenced:
Matthew Loraditch, who worked as a page with Ms. Gallo and Mr. McDonald in 2001 and 2002, said a supervisor had once casually mentioned that Mr. Foley “was odd” and that he later saw sexually explicit text messages that Mr. Foley had sent to two former pages after they left the program.
and this:
But despite Mr. Foley’s warm demeanor, Mr. McDonald and another former page said they later became aware that the lawmaker might have a darker side. Mr. McDonald said he learned that Mr. Foley had sexually explicit Internet conversations with several pages who had left the program. “I was disgusted, but I was not surprised when these revelations started circulating,” he said.
So at least three former pages knew about this before it was publicly revealed (“I was not surprised when these revelations started circulating”).
So once again, I have to ask: How thorough could the "investigation" have been?