In late 2013, CEO Evan Spiegel reportedly rebuffed a $3 billion takeover offer from Mark Zuckerberg.

Now the CEO may be having the last laugh.

Snap shares fell sharply below their IPO price of $17 a share this week, after one of the investment banks that helped take the company public downgraded its stock over worries about Instagram.

The downgrade of Snapchat's parent firm by Morgan Stanley came one business day after Facebook's rival Instagram service introduced what it called its "fun new face filter."

The feature, which allows users to personalize photos with realistic-looking digital elements, was just the latest in a long list of tools Instagram has rolled out to blunt Snapchat's growing appeal to younger users.

In the time since Spiegel spurned Zuckerberg, Facebook has turned Instagram — a photo-sharing service it acquired in 2012 for just $1 billion — into a key growth driver and a Snapchat killer.

And it's done it with product updates — from photo filters to digital sunglasses — that appeal to the same youthful demographic Spiegel used to build up Snapchat.

"The death knell for Snapchat was Instagram Stories," said David Pierpont, vice president of performance media for Ansira, a digital ad agency with more than 100 clients, referring to a video-sharing feature launched last August.

"When we saw that, we said, 'It's over,'" Pierpont told CNBC in a phone interview.

It's not just that Instagram can mimic Snapchat features, Pierpont says, but that when combined with Facebook's own service, it can offer advertisers access to more than 2 billion users, as well as detailed data about their online "likes" and habits.