I was in high school when it was released. It was notoriously ubiquitous. Everyone had one. People said terrible things to each other - it was a free place to trash anyone you hated and really get them down.
Then, there was a rumor that Formspring was a huge fraud and was built as a complete joke and that they'd release the names of all the people who had posted questions on April Fool's. People freaked out and stopped posting, and those who had Formsprings released that they were happier without Formspring.
This is obviously a very microscopic look, but it wouldn't surprise me if Formspring did achieve the numbers they reported. I didn't know any peers of mine who didn't have one, back in the day and the "popular kids" at my school got hundreds of questions a week about everything.
