The floor was in fact made up of secrets that people from all over the country had sent in. Early in the season there was an article about the contest they were holding, here's the link: http://www.wgi.org/news_detail.php?id=1156
And I'm pretty sure the voice-overs were of people telling secrets. My favorite quote from the show, (the only one I remember, lol) is "You can't hold on to the past; you just have to look to the future."
The show was based off of a online community art project called PostSecret, started by Frank Warren, where people anonymously send in secrets on a postcard that they have never told anyone before. I won't incude a link because some secrets that people send in are sometimes a little... well... not family-friendly. But the blog is based on the idea that we all have secrets that we feel we can't let out because we know we'll be judged... so PostSecret is a way to let it all out and get the secrets off of your chest anonymously.
Frank Warren has created four compilations of the millions of secrets that he's been sent over the years, all of which have become New York Times bestsellers.
I've loved the concept of PostSecret since it began and was so excited when I heard that Flanagan's show was going to be based upon it!!! That said, I think they pulled it off rather well.
Just a little bit of background info for the uninformed