NEW YORK » Instagram, the popular photo-sharing service that Facebook bought this year, is the target of a storm of outrage on Twitter and other sites after a change in its user agreement hinted that it might use shared photos in ads.

It’s not clear that anything substantive has changed in Instagram’s new terms of service, which were posted Monday and go into effect Jan. 16. As is the case before, the service reserves the right to use shared photos in any matter it likes, though the photographers keep “ownership” of the photos.

The updated terms of service say users agree that their photos could be used “in connection with paid or sponsored content.” The current terms say the service can place ads “on, about or in conjunction with your Content.”

The fast-growing site is a popular way to share photos from cellphones. Facebook Inc. bought Instagram in September. The cash-and-stock deal was worth $1 billion when it was announced in April, though that fell to about $740 million by the time it was completed because of Facebook’s falling stock price.

The updated terms suggests that Facebook wants to integrate Instagram into its ad-serving system, which can, for instance, promote an item by telling users that their friends “Like” it. The new terms make it clearer that Instagram could use your photos to market to your friends.

Instagram announced the change in a blog post, but didn’t explain its intentions. It said a concurrent change to the privacy policy, a separate document, would help Instagram function more easily as part of Facebook.

Twitter users were vowing to cancel their Instagram accounts in response to the policy change, complaining that the new terms would essentially let the service sell people’s photos for ads.