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Facebook also has an ad service. Social network users contribute voluntarily too.

The difference is that the vast majority of users are unaware that their content is making other users stay within Facebook as long as possible and feed it with more data to make money for Facebook alone.

Most people pick Adsense because it’s the de facto service to use. Name a competitor to Adsense for your average small site that doesn’t take a lot of work or pay a lot less in aggregate. It’s not the same as truly voluntarily using it.

Adsense isn’t Google’s main revenue or profit source either. That would be AdWords and specifically ads on their own properties.

Google Maps is community driven as well. I contributed to it, then I was like "wait, why am I not contributing to OSM instead?" Google does give rewards for the contributions but they're laughable compared to the amount of time you put into the work.

Their main revenue is actually Adwords.