By definition, ad tracking (cookies etc...) is opaque to most people and explaining it publicly would make it seem like FB is doing something more nefarious than others.
It's an important discussion but his response would just make FB seem way worse on that specific issue than highlighting how the entire ad network ecosystem works.
Even those of us without FB accounts (with forgone networking opportunities) are at risk because we all have friends without the same cynicism.
The only ad network in front of Congress today, was Facebook. 'Everyone else is doing it' or 'this is the nature of the business' doesn't qualify as an excuse. Perhaps it could have been good for the large public to know what goes on with ad networks these days.
Any legislation that comes out of this hearing wouldn't apply only to Facebook. Facebook admitting the extent of its tracking could help clean up the "entire ad network ecosystem."