The couple took a vacation to Shanghai back in March, and they must have happened to cross paths with this documentary film crew. While the media did hound Zuckerberg and Chan throughout their visit, this brief appearance seems to be completely accidental.
Naturally, it's blown up across social media sites. On China's Twitter-esque Sina Weibo, users have been posting screencaps of the Zuckerberg-Chan cameo. No word yet on whether Zuckerberg "likes" this, though he's likely less excited about this random occurrence than the rest of us.