A new EU law imposing stricter online regulation comes into effect on Wednesday and the biggest platforms like Facebook and Google will have until February 17 to reveal their user numbers.

The Digital Services Act (DSA) rules will be fully applied 12 months later from February 17, 2024, but officials will need time next year to decide which tech giants are big enough to need close observation.

The DSA was designed to combat online hate speech, disinformation and piracy, in Europe at a time when much of the internet content seen by EU citizens is controlled by US-based companies.

Under the new law, all social media platforms, online market places and search engines will be obliged to react more quickly to remove content deemed in breach of EU regulations.

This will include measures to limit the use of sensitive private data in targeting ads at European users and will insist on more transparency for the algorithms that suggest content.

But the new rules will come into effect earlier for what Brussels calls Very Large Online Platforms (VLOPs) and Very Large Online Search Engines (VLOSEs) -- those with more than 45 million active users in the EU.

At current user numbers, this definition would hit around 20 firms, including Meta and its social networks Facebook and Instagram; Google and its video platform YouTube; and iPhone-maker Apple's platforms.

The micro-blogging platform Twitter, recently bought by entrepreneur Elon Musk, will almost certainly also be included, along with China's video-sharing platform TikTok, German retailer Zalando and Dutch hotel site Booking.

Any site that could be big enough to make the cut must publish its European user numbers by February 17, 2023 and the DSA regulations will come into force once the European Commission has confirmed their size.

This implies that, for the giants, the DSA's rules -- stricter for bigger platforms -- could come in in late 2023 rather than in February 2024, when they will apply to all.

The VLOPs could be fined sums equal to up to six percent of their global revenue or even be banned from the huge EU market in the case of serious, sustained breaches of the rules.

The DSA complements another new EU law, the Digital Markets Act or DMA, which prohibits anti-competitive behaviour by the so-called "gatekeepers" of the internet and went into force in November.