Maas told German newspaper Handelsblatt (link in German) that both platforms are too slow to respond to complaints and requests to remove offensive posts.
The sites respond "relatively quickly" to requests from formal organisations but tend to ignore ordinary users, Maas told Handelsblatt. Facebook posts are only removed after complaints from users 46% of the time, and only 1% of Twitter posts are taken down, he said.
The sites must change their practice or be made liable, Maas said: "This sword of Damocles is hanging over social media, and they still have time to prove that we don’t need to make it their statutory obligation."
Microsoft, Twitter, Facebook and YouTube said in June that they will take steps to prevent hate speech from spreading online under a new code of conduct agreed in partnership with the European Commission.
Under the voluntary code (3-page / 108KB PDF) the technology giants said they would review the majority of requests to remove hate speech in less than 24 hours and to "remove or disable access" to that content if necessary.
The code will require the four businesses to have dedicated teams in place to review requests for hate speech to be removed. Those teams will review those requests against their own rules and guidelines as well as, where necessary, legislation designed to combat expressions of racism and xenophobia.
Maas also told Handelsblatt that an investigation into US technology companies is "overdue".