"... no reason to doubt Zuckerberg honestly to do good."

That is not the way making a claim works. A claim is made, and evidence to support that claim must be brought forward before the claim is to be accepted. We have a claim, but no evidence for that claim in hand, and as such, intellectual honesty dictates that our doubt is 100%.

There is the old yet reliably sourced claim of Zuck calling his users dumb fucks. It is probably that his opinion has changed as he has matured, however the sourness of the original intent remains a part of his Facebook website.