I believe the problem last time was that whomever it was who was doing the answering (FB lawyer?) too often claimed not to have the info being asked for ("I'd have to check and get back to you"). The questioners felt it was an intentional ploy to weasel out of answering uncomfortable questions. Hence the current insistence that Zuck be there himself.
There's no guarantee, in fact I'd suspect it's less likely, that zuck would be able to answer those questions better than a relevant lawyer or more relevant lower-chain director or vp.
