Is the answer

>! www.roblox.com

See [Wikipedia](http://en.wikipedia.org/wiki/Roblox) or [the site itself](http://www.roblox.com) for why this fits the clues:

>! people use it to play and develop games (development "using a sandboxed edition of Lua 5.1"), and many users are signed up to it.

<? (I got this answer by just looking at the OP's user profile page!)>