Starting in January, Instagram users might be surprised to find their photos or faces starring in advertisements when they log onto the photo-sharing service or are browsing Facebook.

Instagram announced changes to its terms of use late Monday night that gives them the right to share user snapshots in advertising. Most notably, the policy changes say that Instagram has the "right" to use users' photographs, usernames and likenesses in advertisements without notifying them.

The new terms of use -- scheduled to go into effect Jan. 16 -- say that while Instagram does not own any user-uploaded content, users "hereby grant to Instagram a non-exclusive, fully paid and royalty-free, transferable, sub-licensable, worldwide license to use the Content that you post on or through the Service."

And these Instagram photos as advertisements may not be limited to Instagram. With its new terms of use, Instagram has granted itself the authority to share user information with third party services -- including Facebook -- in the new year, meaning someone's Instagram photo could appear as an advertisement on a host of other platforms.

The new terms bring Instagram's terms of service in line with parent company Facebook and are part of a wider push to scale revenue, especially on mobile devices where Instagram is popular. Converting user content created when a post is "liked" by a user and turning that into an ad is a core part of Facebook's ad model.

In a post on the company blog, Instagram said the changes were made to prevent spam and to more closely integrate its service with Facebook.

"Nothing has changed about your photos' ownership or who can see them," the blog post says.

Users may not even know when they're viewing an Instagram is being used as an advertisement, however. Instagram's new terms of use say that while user photos may appear in advertisements, the company does not have to identify a photo as "paid services, sponsored content or commercial communications."

Minors are included in this scheme as well. If a user is younger than 18 years old, he or she acknowledges that a parent has agreed to letting Instagram use his or her content for advertising purposes.

Ian Schafer, CEO of digital advertising agency Deep Focus, said that while the new terms of use may give some pause, users need not be overly concerned.

"I don't think people should freak out," Mr. Schafer said. "The Internet freaks out real easily. This is what happens every time."

Mr. Schafer added that the new terms of use is merely legalese designed to allow Instagram to monetize its service (which is free for users) and that he doubts Instagram will actually sell user photos to other parties.

"They have no interest in re-selling individual photos. I don't think that 's the intention here," he said.