I would argue that the ad, as it is, proves her point well. Political ads tend to make claims/conclusions that tend to be believable or within the realm of possibility.
Nobody believed it though, not even the_Donald. If that was the point it failed quite spectacularly. Hope her campaign tosses it as a silly idea and not make any more fuss around it.
Facebook's reputation wrt political influence is so bad that a claim like this would be much more plausible to the opposition.
It's become a standard refrain in political discussion that [media outlet, platform x] is behaving non-neutrally to help your opponents.
If trump ran a campaign saying that facebook was colluding with warren to manipulate outcomes many trump supporters would believe it-- perhaps it was facebook sucking up to prevent being split up-- and many warren supporters would disbelieve it.
If I heard that Mark Zuckerberg endorsed Trump's reelection from someone I trusted (rather than from a political ad), I would have absolutely believed it. ... but then again I don't particularly care for either facebook or trump.
At the end of the day the Warren campaign has effectively admitted that its willing to run overtly false adds targeting companies that they've previously taken issue with. They had a justification for doing so, but that justification may ring pretty hollow for people who are undecided in supporting the campaign.
Trump supporters often justify many of trumps' more obviously untruthful statements with excuses like you're not supposed to take his words literally. Unsurprisingly, many people are not convinced by this position.