Building big data? Are you building a security headache too?

The world and its dog has been shocked by the Prism news story. Early in June, we found out that the US National Security Agency (NSA) had developed a secret data-gathering mechanism to steal all our data and store it in a large data warehouse.
We are outraged that it is being mined, searched and otherwise prodded. But do we really think that big data security problems stop at Google, Facebook, Microsoft and Fort Meade?
The private sector has been collecting data on all of us for ages. It is stored in massive data sets, often spread between multiple sources. What makes us think this is any more secure? At least the NSA is well trained in keeping it all under lock and key.