Facebook just tried to spend $3 billion on a 20-person company that lets you send disappearing photos. At least, that's the word from The Wall Street Journal, a rather trustworthy source.

According to the paper, SnapChat rejected the offer. But the amazing thing is that Facebook would offer that much money in the first place. SnapChat has no revenues, and its collection of users – however many there are – is puny when you consider that Facebook reaches over 1.2 billion people around the world. Across the internet, so many people are asking themselves: Why on earth would Facebook offer so much for this tiny company?

There are some very good answers to that question – but not good enough to justify a $3 billion acquisition.

The short answer is that photos are the fuel that social networks run on, and Snapchat processes nearly as many photos every day as Facebook itself. The two-year-old service lets you take photos and videos on your smartphone, annotate them, and send them to friends. The twist is that your friends have only so long to view them. You can put a detonator on each "snap."

As pointed out in an excellent analysis from blogger Benedict Evans, Snapchat claims to process around 350 million photo uploads per day (as of September), the same rate reported for Facebook – though Facebook gets an additional 55 million daily uploads via Instagram, the photo service it acquired for $1 billion in April 2012. When Facebook bought it, Instagram was about one-fifth its current size. As Evans says, Facebook timed its acquisition well, right before Instagram exploded. With its purported offer for Snapchat, Facebook no doubt anticipates a similar trajectory. If Snapchat has 350 million daily photo uploads today, imagine what it could have in three years.

Facebook has an especially keen appreciation of the emotional power of photos. The social network was born after co-founder Mark Zuckerberg began an often controversial quest to place photos of his fellow Harvard students online. By making photos a priority, Facebook became the dominant photo-sharing site of the desktop era. It seems determined to extend that dominance into the mobile era, an era characterized by an explosion in both photo-taking and photo-sharing services.

In addition to helping Facebook corner the market on mobile photos, Snapchat is also popular with teens, a group with whom Facebook has struggled to connect.

But that doesn't mean a $3 billion offer makes sense.

Unlike the photos that were uploaded to Flickr in 2003, or to Facebook in 2004, or even via Instagram in 2012, Snapchat shots are not meant to be widely shared or to be retained for any length of time. They have some value to Snapchat, in that they generate traffic through the service and create opportunities to show ads, but it’s hard to see how they do much to enhance Facebook.

Indeed, for teens, much of the value in Snapchat is precisely its distance from Facebook: If you’re friends with your parents and relatives on Facebook, you don’t want to share your most candid pictures there. Snapchat is a safe space away from the judgmental eyes on Facebook, which in turn makes it a potentially dangerous place, which in turn makes it fun.

The extent to which Snapchat can be integrated with Facebook is really the extent to which it can be ruined for many of its users. If Facebook tries to get billions of dollars in value out of Snapchat, it may well ruin the product in the process. Snapchat understands this dynamic better than anyone. And that's probably why it gave Zuckerberg the big "no."