They don’t have to sell the information to their partners. They could, for example, buy information associating last names and ZIP codes with estimated income levels, and then tell advertisers, “if you pay us $X we will put your ad on those Google+ pages where, according to our statistical estimate, at least 75% of the readers make over $100K.” The advertisers wouldn’t actually know the names of the people that Google forwards the ad to; they would just get the clicks.
There are a number of ways for Google to do that based on network, searches and other information that give them a high statistical probability of being right without having to go through the trouble of actually matching names with lists. That's a capability that goes right into retargeting/remarketing lists and certain ad servers.
