My girlfriend later confirmed that there are indeed lots of websites using just numbers as their domains. She told me this is sometimes used as a way to hide websites, mostly gambling and porn, and in some cases even sell access to them by just getting money and giving the secret domain name in exchange.

I would be slightly disturbed if my girlfriend knew about secret gambling and porn websites.

When I hear numerical domains, the first thing that comes to mind is the website of Intel Open Source Technology Center: https://01.org/