For some examples of people using Snap in the real world, check out http://snapframework.com/faq#is-anyone-using-snap-in-product.... The guys at Silk (http://silkapp.com) use Snap for some of their back end infrastructure. Janrain discussed their use of Snap in this presentation http://vimeo.com/21210266. In fact, when you log in to ladygaga.com a Snap server is involved at some point. In addition to those, I and others have used Snap to build non-trivial proprietary websites.
Snap intentionally has not made ORM/persistence decisions. The developer is free to choose any Haskell library (including the one used by Yesod) to interface with a database. There is a library written to simplify the use of mongoDB with Snap (pre-0.6). The new snaplet abstraction introduced in 0.6 makes writing these kinds of libraries much easier and provides a clear path to follow, so I imagine we'll be seeing a lot more activity in this area. In fact, someone has already released an HDBC interface as a snaplet. They even have a tutorial for it here: https://github.com/norm2782/snaplet-hdbc/wiki/HDBC-Snaplet-T....