Snap's data has real value, like Facebook's. For most of their users, they have a large portion of their phone's contacts (whether you opt in or not is irrelevant due to your contacts opting into this + a network effect.) They also have media preferences based on what articles you're watching/reading. They also know who you're most interested in (who's stories you're watching most, and who you send/receive snaps from the most) and what their interests are, so they can potentially infer interests from that too, even if you don't read articles. They have location data turned on for most of their users thanks to geo filters, and if they've been "good" data miners, they're sending your facial mapping home when you do a face filter effect. I'd be surprised if Snap doesn't have the most comprehensive faces database out there with all the angles they get from selfies (though they may not have the most unique faces.)
Oh yeah, and they also have the perceived privacy that Facebook doesn't have. Deleting messages doesn't actually delete them, but it just feels better when they're gone, so people are more candid on Snapchat.
Not to mention they've also done an excellent job monetizing their features thus far, with very little intrusion that I've noticed as a user. You get the occasional ad when you're looking through stories, but most of the money is coming from things like sponsored content and geo filters, which users don't complain about.