By requiring hardware attestation, like Private Access Tokens in iOS/MacOS (see https://blog.cloudflare.com/eliminating-captchas-on-iphones-... for example) and Web Environment Integrity and its successors ( https://github.com/explainers-by-googlers/Web-Environment-In... ).

According to the existing law, Facebook can't provide service to any user whose OS doesn't say they're old enough. According to this new law, Linux won't say that. So Linux users won't be allowed to use Facebook.