derekullo, if he'd mentioned one of those, not owned by Facebook and actually competing with what Facebook is providing, the (natural) follow up question would be how big that service's active market share is compared to Facebook. The number would most likely not even register on a decimal percentage scale. That would automatically put Facebook in some sort of antitrust issues...

He would almost certainly get those questions if he does a hearing in the UK. Which is why he doesn't want to do that.
The numbers for Google+ is massively inflated as it's pretty hard to activate an Android device or Gmail account without also getting that Google+ account. No one uses Google+. Even Google themselves are more active on Facebook than their own social network.