Direct questions hover around 1/1. Questions sent to followers, 5/1. It’s probably a higher engagement, albeit different, than Tumblr/Twitter/FB.
When you ask a question on Twitter, how many of your followers respond? Very few. The nature of both sites is obviously different, but the engagement on Formspring remains pretty good.
Also, there’s only so many times you can answer a question about your favorite movie, or where you met your husband/wife before it gets boring. Original questions (with photos, links, tags) get a far higher response rate.
Me asking 1Q to 5 users results in 5Q being delivered to the respective users. Out of 5, and depending on the quality of the question, the level of activity of these users, generally results in 1 to 2 responses.
Obviously the engagement is specific to the user asking the question. Some popular users easily reach a 75% engagement rate.