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.
Thanks for catching this! I fixed it. What's remarkable is that apparently a lot of people don't want to believe that this isn't Vonnegut, which is why it slipped past me when I did a web search on it earlier.
I've tried contacting multiple places that have published this quote to have them retract it with no results. I've also contacted Snopes twice with no reply.
I was once contacted by someone who wanted to know my source for the quote because they were tracking Vonnegut's archeology experience in his biography and wouldn't believe me when I said it was about my own life.