You can also make a game if you're good at scripting, business and values and building. It has a system to reward to give back to builders that make games to keep roblox running.
It has a total of 10M+ players and pretty populated. I sell graphics and have made dons of virtual money on it.
ROBLOX has a system where you can turn virtual money into USD, and its a program to prevent black marketing and RWT such as in Runescape.
100K Robux [Virtual Money] can be turned into $250 USD. You get the rates.
I'm a graphic designer and can make 100K robux in 3 days due to popularity of graphics and people willing to buy graphics and make their game better.
Good luck, ask questions here if you need.
This used to my portfolio for selling graphics: www.deviantart.com/cerualen/gallery