The 1 billionth Roblox user is widely reported to be Azoo5573. A Roblox- related post from 2019 says Roblox hit 1,000,000,000 users and identifies that account as the 1 billionth user.

What people mean on Reddit

Reddit discussions about the “1 billionth Roblox user” usually point to the same username, Azoo5573, and joke that the account may have been created by a bot or quickly taken over after creation.

About the Robux angle

There is no reliable evidence in the sources I found that the 1 billionth user was awarded Robux as a special prize; the claim is mainly about the username tied to that milestone. The Reddit chatter seems to be more rumor and lore than confirmed reward info.

Context

A later Roblox-related video also repeats the idea that Azoo5573 was the 1 billionth user and says the account was password-guessed early on, which fits the broader community story around the account.