We may share User Content and your information (including but not limited to, information from cookies, log files, device identifiers, location data, and usage data) with businesses that are legally part of the same group of companies that Instagram is part of, or that become part of that group (“Affiliates”). Affiliates may use this information to help provide, understand, and improve the Service (including by providing analytics) and Affiliates’ own services (including by providing you with better and more relevant experiences). But these Affiliates will honor the choices you make about who can see your photos.
Those cookies and location data will help Facebook get better targeted advertising on the social network effective ASAP. But, given the massive amount of personal information we already give Facebook, this relationship is likely to turn the other way: Instagram gives Facebook more location, photo data, and "liked" brand pages; Facebook will give Instagram everything it has.
Increased advertising should also means less privacy for users. Instagram reiterated in the blog post that it still doesn't own rights to the images posted through the app — a stance that differs from Facebook's. However, the new partnership only expands Instagram's tracking abilities: it already used cookies to deliver relevant ads on other networks, but now it will share that information directly with Facebook, and vice-versa, thus making the ads more relevant (and valuable). Facebook has yet to say anything specific about its Instagram advertising strategy — other than to say it has one — and that may be precisely because there's a lot more at stake here than some synced-up terms of service.