"He let the officer know that he had a firearm and he was reaching for his wallet and the officer just shot him in his arm," Reynolds said during the livestream.
During the graphic 10-minute video, Castile can be seen with blood covering his shirt. At one point, he moans out in pain before ultimately slumping back in his seat and dying. Reynold's video has been viewed on her Facebook page more than 5 million times.
"The images we've seen this week are graphic and heartbreaking, and they shine a light on the fear that millions of members of our community live with every day," Zuckerberg wrote. "While I hope we never have to see another video like Diamond's, it reminds us why coming together to build a more open and connected world is so important -- and how far we still have to go."
Google's Twitter account also tweeted out a message, saying the company stands "in solidarity with the fight for racial justice."
Days after Castile's death, Facebook headquarters in Menlo Park, California, posted a large Black Lives Matter sign with victims' names written in the text, Fusion reported. Trayvon Martin, Renisha McBride, Jordan Davis, Amadou Diallo, Eric Garner and others were listed.