With its new Stories feature, Facebook's(FB) - Get Report Instagram isn't simply trying to compete with a popular Snapchat feature. It's imitating Snapchat in a way that has led many jaws to drop, and fueled speculation Snapchat might sue in response.

The fact Instagram felt compelled to make such a move, in spite of all its recent success, says a lot about how Snapchat's rise has put social media rivals on edge. It might also say something about how driven Facebook is to boost the amount of content its users share.

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Instagram's Stories, much like the Snapchat feature sporting the same name, let users share photos and videos that disappear after 24 hours. They won't appear in a user's normal content feed or profile grid, but rather in a bar at the top of Instagram's app. As with Snapchat Stories, there are no likes or public comments. Notably, if a user's posts are normally visible to all Instagram users, the same will go for his or her Stories by default.

Instagram says it was motivated to launch Stories by the fact many of its users want to share content that doesn't stick with them forever, and -- with many users often deleting posts that don't get enough likes -- that a lot of them want to share content for which they don't have to worry about how much positive feedback it received. "We need to have a place where you feel free to post whatever you want without the nagging fear of, did someone like that or not," says Instagram chief Kevin Systrom.

The fact Snapchat has been seeing runaway growth with the help of Stories and other features that enable short-term sharing can't be lost on Instagram either. Bloomberg reported in early June Snapchat's daily active user (DAU) count had risen to 150 million, up from 110 million in December and above what Twitter is estimated to have. The company is also now getting serious about monetizing its content, and in doing so becoming a greater rival to Facebook and Instagram for social ad dollars.

Instagram has been growing rapidly as well: It topped 500 million monthly active users (MAUs) in June, up from 400 million last September and 300 million in December 2014. But its engagement rates appear to be slipping: While MAUs rose 67% from December 2014 to June 2016, the number of photos shared per day rose only 36%, to 95 million. Likewise, a study by social analytics firm Quintly found the average number of interactions per Instagram photo and video posts respectively fell 27% and 39% annually in the first quarter.

Parent Facebook has also been dealing with declining public sharing of original content (as opposed to content produced by third parties), and recently updated its news feed algorithm to show more original material. In April, The Information reported internal Facebook data showed a 21% annual decline in "original broadcast sharing" as of mid-2015, and a roughly 15% drop as of early 2016.

Some of this sharing is moving to private messaging platforms such as Facebook's Messenger and WhatsApp. But some of it is also going to Snapchat, which lets users share without having to worry about likes or a permanent digital record of what they've shared. By copying Snapchat so blatantly, Instagram is letting the world know just how seriously it takes this threat.