[erebus_rex]

Could someone explain why this would dilute voting power?

Voting power of an investor = votes held by investor / total votes

While the dividend will slash the economic ownership of my class A stock, that stock still has the same voting power because the number of votes and distribution of those votes has not changed.

Am I missing something?

[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.

[ThrustVectoring]

It's not the split that dilutes things, it's Mark Zuckerberg's sale of non-voting stock. The split changes nothing - the sale means that non-Zuckerberg FB shares have less voting rights per dollar invested. (Basically tautologically, since Zuckerberg's shares have more voting rights per dollar of value).