Social media apps of Instagram and Snapchat.

Facebook has been trying to freeze out Snapchat for years.

The social media giant reportedly tried to buy the then fledgling start-up for $3 billion a little more than three years ago. After that failed, Facebook bought photo filter app Masquerade to mimic Snapchat's lenses feature. More recently, Facebook launched Instagram Stories, short videos and photo posts that disappear within 24 hours that are very similar to Snapchat's main offering.

And while Instagram Stories has been a success — it reached 150 million daily active users in just five months, a feat that took Snapchat five years — download and usage data from App Annie shows that Facebook hasn't won the fight yet.

And, with Snapchat valued at up to $25 billion and about to go public, it's probably too late to put the company away decisively.

For one thing, Instagram Stories' growing user base hasn't impacted Snapchat's popularity. Both global and U.S. download rates for Snapchat and Instagram have remained relatively the same over the year, according to App Annie.

Also, people who use Snapchat use it a lot. App Annie said that Snapchat users averaged 330 sessions in December 2016, compared with Instagram's 160 average sessions per user. (To be fair, Instagram session lengths were almost twice as long as Snapchat's, which cancels out Snapchat's advantage to a degree.)

Perhaps most importantly, Instagram Stories hasn't diminished advertisers' growing enthusiasm for Snapchat, according to multiple media buyers.

While Instagram Stories has added features to entice advertisers — like detailed analytics on who posts reach, as well as ways to target specific users — it still doesn't have an answer to certain kinds of Snapchat ads that are considered core to the company's offering.

For example: Companies on Instagram Stories can't place their ads next to publisher content like they can on Snapchat Discover, or sponsor photo effect filters, which Snapchat calls "lenses." While Facebook has acquired technology to create lenses, it has yet to integrate it into its platforms.

"I don't think [Instagram Story ads] replaces the ad formats of sponsored lenses on Snapchat," David Eisenman, co-founder and CEO of advertising agency Madwell. "It's a highly engaging unique to Snapchat format."

However, there are some concerns Instagram Stories may steal from Snapchat going forward. Instagram Stories has secured big clients including General Motors, Buick and Nike, social data analytics Listen First Media's Jason Klein said. In addition, Instagram Stories' ability to add users so quickly shows it's easier to on-board existing users to a new feature than to add people to a new app, he said.