But how is it spying if you use the service voluntarily, they tell you exactly that they collect and often (always?) let you opt out of the collection.
Well, consider spies in WW2, specifically the "loose lips sink ships" variety.
The spies that saying warns of were not crawling around in vents with night vision goggles, they were your friends, neighbors, and coworkers who would relay semi-public information to a processing center that used the info towards harmful ends.
I see this situation as a direct analogue. We communicate naturally with a supposedly disinterested or trustworthy party, who then collects that information and distributes it to others, without our best interests in mind.
That's a really diluted standard for "spying".