Old StumbleUpon Question

For those of you who remember the popular website StumbleUpon, I have a question.

The top of the page was the "stumble bar" and beneath it was the random website. How was this random website loaded? Was it in an iframe?

Any details as to how sites were displayed are appreciated!

Thanks!

It was iframe.

Man, stumble upon was a great traffic source. Their ad network was like a high-quality PPV network. I made bank on that. Now they have mix.com which seems pointless to me.