In a joint statement published Tuesday alongside progress reports from the companies, the EU said the impact of the "self-regulatory measures" remains unclear.
"Large-scale automated propaganda and disinformation persist and there is more work to be done under all areas of the Code," EU Commissioners Vera Jourova, Julian King and Mariya Gabriel said the joint statement. "We cannot accept this as a new normal."
Social media platforms like Facebook, YouTube and Twitter have faced backlash from lawmakers around the world for failing to contain the spread of fake information in election campaigns.
The EU acknowledged tech companies have taken steps to be more transparent. In its report to the EU, Facebook said it removes millions of fake accounts every day. Earlier this month, the social media giant announced it had removed four networks of fake accounts tied to Russia and Iran.
In a statement to CNBC, a Twitter spokesperson detailed the company's efforts to tackle platform manipulation and said it discloses data relating to issues such as legal requests and rules enforcement in bi-annual transparency reports.
A spokesperson for Facebook said the company is committed to working together with "government, industry, news publishers, and our community." "We appreciate the Commission's extensive report, and share the same commitment to reduce the spread of online misinformation," the statement said.
Still tech companies could soon face stricter regulations in the EU related to disinformation and illegal content online. European Commission President-elect Ursula von der Leyen has said she would prioritize a new Digital Services Act within her first 100 days in office, which would upgrade "liability and safety rules for digital platforms, services and products."
Proposals from the European Commission, the EU's executive arm, then need approval by the different European governments and the European Parliament, a process that can be long and complex.
In addition to the annual reports published Tuesday, the companies published monthly reports detailing their efforts to fight fake news in the lead up to the European election. The EU said it will present a comprehensive assessment of the companies' reports in early 2020.
"Should the results under the Code prove unsatisfactory, the Commission may propose further measures, including of a regulatory nature," the Commission said.
"As a founding partner with the European Union on its Code of Practice on disinformation, we're proud to mark a year of progress since we signed the Code: expanded policies, products and resources dedicated to thwarting disinformation and other forms of attack on the integrity of our systems," said Milan Zubíček, manager of government affairs and public policy at Google, in a statement to CNBC.
— CNBC's Silvia Amaro contributed reporting