Ten weeks after Facebook Inc. pledged to fight vaccine misinformation, such content remains widely available across its platforms as the social-media giant grapples with how aggressively to limit the spread of hoaxes and deceptions.

Facebook as of this week is still running paid ads for a prominent antivaccination group that suggests unethical doctors have conspired to hide evidence of harm vaccines do to children. Both the company’s main platform and its Instagram app recommend additional antivaccine content to users who...