For starters, Instagram will be able to share user information with Facebook, its parent company as well as external affiliates and advertisers. This lets the advertisers in Facebook's network use information that users have shared on Instagram (such as favorite places, hobbies and more) in an effort to improve how it targets ads to them. So you're likely to see ads more relevant to your interests, but your personal information will be mined toward that end.
Ready for your closeup? Photos uploaded to Instagram could be used in either an Instagram or Facebook advertisement. Even more troubling; someone who isn't an Instagram user but has a photo of themselves up on the service can be used in an ad if their photo is shared by a friend. So before you snap and share a photo of a friend who isn't an Instagram user, you might want to offer them a quick disclaimer.
Youngsters are also not spared. While Instagram says that people must at least 13 to use the service, if a teenager signs up then they are agreeing that a parent or guardian is aware that their image, username and images can be used in ads. On top of that, ads will not necessarily be always labeled as such.