Yangqing Jia, a director at Facebook involved in artificial intelligence, left the company this month to take a role as vice president of engineering at the Alibaba Group, according to his LinkedIn account. Mr. Jia joined Facebook in 2016 and lead teams of AI researchers and engineers working on ads, news feed, search and other efforts. Facebook did not respond to a request for comment. It isn’t clear what prompted his departure. But Facebook needs all the AI talent it can get as it looks to use the technology to solve some of its most vexing problems, including policing content on the service and pick out fake accounts.