The announcement was made Wednesday on behalf of the couple during a ground-breaking ceremony for the Ravenswood Family Health Center at 1885 Bay Road.
The Zuckerberg/Chan donation pushes the project over its fundraising goal for the construction of the facility, though another $7 million is needed for equipment and furnishing.
The pair joined many Silicon Valley individuals, businesses and organizations that have contributed to the $29 million project.
Construction of the 38,000-square-foot facility could begin in a month, pending approval of permits by the city, Ravenswood Health Center CEO Luisa Buada said. The center is expected to open in spring 2015.
Since 2001, Ravenswood has provided comprehensive medical, dental and mental health services in East Palo Alto out of portable buildings at 1798-A Bay Road in the city's industrial zone, surrounded by car repair and sheet-metal shops. About 97 percent of the health center's clients are ethnic minorities and 93 percent live at or below the poverty line, East Palo Alto Mayor Laura Martinez said. Of its 150-member staff, 45 percent are hired from the community, making the center one of the largest employers of East Palo Alto residents in the city.
The new facility will enable Ravenswood to serve 22,000 patients per year, double its current 11,000. It will have 58 exam rooms, 13 behavioral-health counseling rooms, a laboratory, imaging and mammography capabilities, optometry exams and a pharmacy.
The center will also enable residency programs in pediatrics, internal medicine and family practice to expand, Buada said.
The announcement and ground-breaking was met with cheers and applause in a white tent at the new site, which is near Pulgas Avenue.
The new facility is a far cry from the days when pediatric vaccinations were conducted from the back of a pickup truck, said Chris Dawes, president and CEO of Lucile Packard Children's Hospital and a member of Ravenswood's advisory council.
It was 1998, and a Packard Hospital pediatric fellow had launched a project to vaccinate East Palo Alto children. Dawes and the student drove around to apartments and offered the vaccinations from the back of the truck.
"It made me a bit nervous. I thought, 'There's got to be a better way to do this,'" he recalled.
He joined the task force that went on to create Ravenswood.
Buada had initially planned to work as a consultant for three months, but over the years she has helped build the Belle Haven Clinic in East Menlo Park, the Center for Health Promotion, which offers classes and programs for clients with chronic illnesses, and Ravenswood Family Dentistry.
"Are you tingling? This is so exciting for each and every one of us," Congresswoman Jackie Speier, D-San Mateo, said at the ground-breaking.
The center received $5 million for construction from the Health Resources and Services Administration, which got the money through the Affordable Care Act, she said.
Congresswoman Anna Eshoo, D-Palo Alto, who has long been involved in supporting Ravenswood, hailed the project as a long-needed community resource.
"This has been the most underserved community in our region and we all knew it, and something had to be done about it," she said.
Major donors include the David & Lucile Packard Foundation, Palo Alto Medical Foundation/Sutter Health, Silicon Valley Community Foundation, John and Sue Sobrato and the Sobrato Family Foundation, John and Jill Freidenrich, Dick and Sue Levy, and Gordon Russell and Tina McAdoo.