[cbr]

    So he proposed setting up a new Facebook stock class.
    The new shares would automatically dilute the voting
    power of existing shareholders, because every share
    with voting power will split into three shares -- one
    that has power, and two that don't.

I think the argument is something like: people knew Zuckerberg would want to get money out of FB, so even though he currently had majority control they expected he would give it up soon. Introducing multiple stock classes allows him to sell stock while keeping majority control, so he's reducing other people's voting power relative to expectations, while technically everyone who had N votes before the split still has N votes after the split.