How does anyone know for sure that all of that traffic (or at least the meta data related to the traffic) is not prepped for data mining on a per-user basis ?

There would be obvious commercial advantage in doing this, just for starters.

Data mining services like facebook gives you a solid picture of the interests and connections that the target person broadcasts publicaly. This is useful information, and it is exploited by any number of parties ... and people are (hopefully) aware of that.

Data mining snapchat gives a deeper picture of a person's interests and connections, as they are broadcast whilst that person believes that this information is entirely private.

Aggregating data from both sources (which would be almost trivial), would provide a very detailed view of the target. Much more than the sum of each dataset alone.

And it matters not whether the owners of Snapchat are completely and utterly above-board with everything .... there is the question of who owns and operates the network that all this info travels over.

Bottom line is that all of that "ephemeral" data that is going through snapchat is no more private than facebook, or youtube, or HAM radio, or telephone calls, or email, or SMS, or pretty much anything electronic.

There is no real privacy with this, but its easy to provide the illusion of privacy.