Spark is a very hot technology, and is relatively young, so it's still very much emerging and changing quickly, but a lot of big people are using it. Amazon, for example, has claimed they're using it, eBay, NASA's Jet Propulsional Laboratories, Groupon, TripAdvisor, Yahoo, and many, many others have too. I'm sure there's a lot of companies using it that don't confess up to it, but if you go to the Spark Apache Wiki page at http://spark.apache.org/powered-by.html.

There's actually a list you can look up of known big companies that are using Spark to solve real-world data problems. If you are worried that you're getting into the bleeding edge here, fear not, you're in very good company with some very big people that are using Spark in production for solving real problems. It is pretty stable stuff at this point.