The video conferencing service Zoom has gained a following in China in recent months. The program lets users have video meetings or make video calls to friends or family. Users in China include people and organizations, from underground religious groups to women’s rights activists. They see the service as a rare window to the rest of the world. But some Chinese fear that window to the world may be closing. Last week, Zoom said it had suspended accounts of three U.S. and Hong Kong activists at the Chinese government’s request after the activists tried to honor the anniversary of the Tiananmen Square killings.