It looks like currently their ads are all player-created, for their own content. For example refresh this page and see the banner at the top:
https://www.roblox.com/catalog?Category=0
But I seem to recall in the past there used to be some regular ads. For example see this archived page:
https://web.archive.org/web/20120427192637/http://www.roblox...
which has this embedded iframe:
https://web.archive.org/web/20120308000433if_/http://www.rob...
which has Adsense javascript embedded in it.