As tech companies mount a “Day of Action” to protest the FCC’s plan to eliminate net neutrality, their campaign notched an important new ally: Facebook CEO Mark Zuckerberg.

In a post on (where else) Facebook, Zuckerberg on Wednesday afternoon called on the FCC to preserve the agency’s current rules, which forbid Internet providers from creating so-called “fast lanes” for preferred websites or delivering traffic from sites at higher speeds than others.

The words Zuckerberg uses are significant since they clearly call on the agency to preserve the current legal regime, known as Title II, which treats Internet providers as common carriers like phone companies. The telecom industry, which is urging the FCC to repeal the current rules, has claimed net neutrality is possible without Title II but most policy experts dispute this claim.

Here are key paragraphs from Zuckerberg’s post (my emphasis):

Net neutrality is the idea that the internet should be free and open for everyone. If a service provider can block you from seeing certain content or can make you pay extra for it, that hurts all of us and we should have rules against it.Right now, the FCC has rules in place to make sure the internet continues to be an open platform for everyone. At Facebook, we strongly support those rules.

Zuckerberg’s declaration is also significant because Facebook in the past has been cagey about its support for net neutrality, and has angered open Internet advocates with its plans for a “Free Basics” program that would provide a limited number Internet services (especially Facebook) to people in developing countries.

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Meanwhile, Facebook’s powerful chief operating officer, Sheryl Sandberg, published a similar message on her own page, echoing Zuckerberg’s words, and directing readers to a site that instructs people how to contact the FCC.

The Facebook executives messages comes as a wide range of companies—from Netflix to Airbnb to Amazon—have posted messages on their home pages, offering support for the current FCC rules.

Despite the broad show of support, the net neutrality campaign faces an uphill battle as FCC Chair, Ajit Pai, is an implacable foe of Title II, and has the votes on the Commission to force through a proposal to reverse it. A final showdown over the FCC plan is likely to take pace this summer.