Nothing had a billion users; the Internet didn't at the time.

MySpace and Friendster both spent significant time as the #1 social sites. Facebook unseated them rapidly. The same is possible for OpenAI.

It had more than Facebook when Facebook launched so I’m not sure what your point is

Dude, if you seriously believe OpenAI has 1B active users, you should go touch grass. Actual estimates are 100-200 million, about a magnitude lower.