Getting started with Google Voice

For a long time, a cell phone was the way to make long distance calls for "free".  Well, free as in I've bought a bunch of minutes that are not tied to a particular destination, and I've paid in advance, so it's cheaper than my Costco long distance plan on the land line.  Now, however, I'm running out of those minutes and am looking to my landline for some of my calling needs.  Google Voice has come to my rescue.

Here's how it works:  I created a Google Voice (GV) account with my cell phone as the primary phone, and only phone enabled for incoming calls.  The landline was added as another phone.  When I want to call someone, I use the GV website to call them.  The calling process is that GV first calls a phone I specify (i.e., the landline) then connects it to the called party.  As far as both ends are concerned, it's an incoming call.  Cell phones may eat minutes for incoming calls but landlines (at least, as far as I know about my own billing plan) don't.  

I signed up for GV to have a "business" phone number I could use that's independent of the numbers I'm otherwise assigned (cell phone, landlines, etc.) and for the useful features. Google Voice has lots of useful features — call screening, assignable voice mail messages, voice mail transcription, ring to more than one phone — but this one will save me real money. 

How does Google make money with this?  My wife says Google is selling information about who I know — mapping my networks by monitoring who I call and who I email.  Probably. Is that worth something to someone?