You are probably right when you say that they'll monitize it (duh, it's a business). And the way google makes money is exactly by harvesting loads of data. Then their search engine (which is implemented troughout every Google feature) will get better. Then people will use it (google products) more often and stick to it. Then they'll make more money.
So stating that it's naive to think that they do it for the data is not entirely correct.