ETA: Okay, I've recently found out that this quote is not Vonnegut, although he did write something similar to this in a letter to schoolchildren. That letter was posted to three--ring's blog with her commentary posted below. Her commentary was copy-pasted to Facebook as being written by Vonnegut, and it's been passed around that way ever since. Proof here.

I'm fixing this post with proper attribution. I did do a quick web search before posting this, but apparently it was too quick. Bleagh.