BRUSSELS—The European Union’s executive body and other national consumer authorities on Friday gave Facebook Inc., Twitter Inc. and Alphabet Inc.’s social networking channel Google+ one month to address scams and fraud on their platforms and bring their terms in line with the bloc’s consumer rules or face enforcement action.

The move by the European Commission and the national consumer protection agencies is an attempt to pile pressure on the companies to change their business practices and make them more accountable in the EU by ensuring consumers can take the firms to court in Europe.

Terms and conditions should be “drafted in plain and intelligible language” so consumers are informed about their rights, the EU said, adding that companies cannot change them without sharing their justification and giving users the option of canceling their contracts.

The EU’s warnings over terms and conditions pertain to contracts such as payment agreements when users buy apps or other add-ons through social media services. Companies should give users the right to withdraw from an online purchase, for example, the EU said.