There are many reasons to like Facebook, Inc. (NASDAQ:FB) stock, but Wall Street thinks one of the biggest reasons is Instagram.

Wall Street firm KeyBanc recently released a positive note on Facebook stock that pointed to Instagram as the stock’s next big growth driver. The argument’s meat is that the core Facebook ad business is maxed out, but Instagram is still in the hyper-growth phase. Thus, as Instagram user growth and ad rev continues to ramp, the photo-sharing app will be the primary contributor of incremental ad revenue into the foreseeable future.

There is nothing terribly novel about this thesis. And I largely agree with it. The core Facebook app is maxed out in terms of users and digital ad real estate, and total available ad real estate is actually shrinking — thanks to management’s push to make the platform less promotional. Thus, the core Facebook app’s advertising business is tapping out.

But, the opposite is true for Instagram. User growth is robust and has a long runway ahead of it, considering the user base is just 800 million. Ad real estate volume is only increasing. And the platform is only growing in popularity among younger users.

So is Instagram a reason to buy Facebook stock?

Of course. Instagram will be Facebook’s big growth driver over the next several years.

Here’s a deeper look.

Instagram Has Big Growth Written All Over It

No matter how you look at it, Instagram has huge growth written all over it.

Previously, social media was word-heavy platforms where people would share life events through various mediums — but the focus almost always on words. Think Facebook and Twitter Inc (NYSE:TWTR).

The future of social media, though, is very different. Social media is becoming increasingly visual-first — users are communicating with one another through pictures and videos. Captions are sometimes included, but the focus is almost always on the visual. Think Instagram, Snap Inc (NYSE:SNAP), and YouTube.

It should be no surprise, then, that the three most downloaded apps in the App Store are YouTube, Instagram, and Snap, or that a recent Pew survey found that the three most popular apps among teens are YouTube, Instagram and Snap.

This is the era of visual-first social media, and in that era, Instagram is king.

Instagram is a photo-sharing app. As such, the platform is at the heart of visual-first social media. But more than that, the platform has pivoted into Stories, and is now the biggest Stories platform in the world. Even further, Instagram has become an ideal forum for brands and influencers to reach their audience through stimulating visuals.

Because of this, Instagram is staring at multiple years of huge growth ahead of it. Advertising dollars continue to shift en masse from traditional mediums to digital mediums. This shift will continue to play out concurrent to a shift in social media engagement from word-heavy platforms to visual-heavy platforms. As such, a majority of those ad dollars will flow into visual-heavy platforms.

Considering Snap’s blunders recently with its redesign and YouTube’s struggles with ad placement, it looks like a majority of those ad dollars will flow into Instagram for the foreseeable future.

Messenger and WhatsApp Are Next

Over the next several years, core Facebook ad revenue growth will slow, and the big driver of Facebook stock will be Instagram ad revenue growth.

But it won’t be the only driver.

Facebook owns social messaging apps Messenger and WhatsApp, both of which have over a billion users. Neither have been monetized yet. Thus, over the next five to 10 years, core Facebook ad growth will stall out, Instagram will turn into what core Facebook is today — a digital advertising powerhouse — and WhatsApp and Messenger will turn into what Instagram is today — a nascent digital ad businesses with hyper-growth potential.

From this perspective, I don’t realistically see Facebook’s ad revenue growth slowing all that much over the next several years. The current dynamic of one big ad platform with slowing growth and one small ad platform with accelerating growth will persist for the foreseeable future.

If so, then today’s 25-times forward multiple for Facebook stock makes no sense. Revenue growth should be able to stay in the 40-50% range, a level which easily warrants a 30 or bigger multiple. A forward multiple around 25 is just way too cheap.

Bottom Line on FB Stock

You don’t buy and hold Facebook stock exclusively because the company owns the world’s most used social media platform.

You buy and hold Facebook stock because the company owns the world’s most used social media ecosystem. That ecosystem includes four of the most widely used social platforms in the world, two of which have yet to be monetized and one of which is still in revenue generation infancy.

As of this writing, Luke Lango was long FB and SNAP.