For its entire life as a public company, Snap (NYSE:SNAP) has been plagued by a relentless onslaught from Instagram. Indeed, one could argue that SNAP stock’s collapse from $30 to $6 was mostly due to Instagram stealing its thunder.

But, SNAP stock investors shouldn’t be so worried about Instagram. Instead, another Facebook (NASDAQ:FB) property poses a bigger risk to SNAP stock: WhatsApp.

WhatsApp is a global messaging app with huge international reach. The platform has already copied one Snap’s two core features, Stories, and has grown that feature — called WhatsApp Status — to 450 million users. That makes WhatsApp Status the largest Stories platform in the world. It’s also more than twice as big as Snapchat entire user base.

But, WhatsApp hasn’t jumped into ephemeral photo messaging yet, which is close to just direct messaging Stories. This is Snap’s bread and butter.

As a messaging app, WhatsApp could easily replicate this feature. If they do, they could kill Snap’s international growth prospects, much like Instagram killed Snap’s domestic growth prospects.

As such, while the drop in SNAP stock from $30 to $6 was because of Instagram, further weakness from $6 will likely be due to WhatsApp.

Snap’s Last Line Of Defense Is Messaging

Snap invented the future of social media with Stories. Unfortunately, they didn’t protect that invention. Everyone else copied it, and now, Snapchat is an afterthought in the Stories world. Domestically, Instagram Stories is far bigger and more popular. Internationally, WhatsApp Status takes the cake.

To be sure, that doesn’t mean Snapchat has no value today. On the contrary, people still use Snapchat in bulk. After all, the company still has nearly 200 million daily active users. But, my read on usage based on talking with a handful of consumers who use both Snapchat and Instagram is this: you use Instagram more for posting public Stories because it has broader reach, and you use Snapchat more for personal and group messaging because the messages are ephemeral and it’s so easy to send pictures.

Assuming this to be true, Snapchat has staying power as a messaging service. We are in a visual based world, and Snap has made it super easy to send direct photo messages. No one has come close to rivaling Snap in this regard. None of Facebook’s properties have perfected this direct, ephemeral photo messaging. Twitter (NYSE:TWTR) doesn’t do it, either. It’s a Snap thing.

And because it’s a Snap thing, this is the last line of defense for SNAP stock. So long as Snap maintains this advantage, SNAP stock has potential to grow into its valuation. But, if this advantage withers away, SNAP stock will likely drop in a big way.

WhatsApp Is a Big Threat to Snapchat and SNAP Stock

The scary thing about SNAP stock is that this advantage could wither away quite quickly.

Just look at what happened with Instagram and Snapchat over the past few years. Instagram was a billion-plus user photo sharing app that dwarfed Snapchat. All they did was copy the Stories idea and enable the sharing of ephemeral photos and videos.

That move on its own stalled Snap’s domestic user growth. In the two years before IG Stories, Snapchat added 33 million daily active users in North America. In the two years since, they’ve added just 14 million North America users. And, that total user base has shrunk for the past two quarters.

Now, that same thing could happen on the international front thanks to WhatsApp. WhatsApp is a messaging app with a billion-plus users that already has 450 million people using its Stories feature daily. This platform is really just one natural and non-intrusive step away from implementing direct, ephemeral photo messaging, and stealing Snap’s bread and butter.

The big problem here is that of WhatsApp’s 1.5 billion active users, only 20 million are from the U.S. That means nearly 99% of the app’s users are international, so if WhatsApp rolls out direct messaging Stories, that will be a feature widely adopted internationally.

That’s bad for Snap because they have a very small international presence. This is partially due to the lack of a good Android app. Nonetheless, management is hoping that an upgraded Android app will reinvigorate international user growth, which was negative last quarter.

That won’t happen if WhatsApp rolls out direct messaging Stories. Just see what happened with Instagram in the states. As such, WhatsApp is a huge threat to Snap’s international growth narrative.

Bottom Line on SNAP Stock

Even after the big drop, SNAP stock is still richly valued, trading at the same sales multiple as Facebook despite having slower user growth and being wildly less profitable.

Thus, Snap still needs everything to go right in order for SNAP stock to work at these levels. But, there are a lot of problems here that make perfection a tough goal.

As such, SNAP stock looks destined to be weaker for longer.

As of this writing, Luke Lango was long FB.