In the broadest sense, any data that tells you something about the interests, purchases, travels, etc. of the user is data that can be sold for marketing purposes. If Instagram has data that says, say, some of our users vacation in Ireland, they might get Aer Lingus to buy ads. You might also recall that FB floated a plan where Instagram would sell content, essentially acting as a microstock agency, but that users (providers of content) would get bupkis. This didn't go over very well.
We are all familiar with Google's tuning it's ads based on your searches, but it turns out they do this with your gMail account also based on the "content" of the eMail. Watched two gents on one of the major networks test this and the test verified this practice.