Problem to me is they seem more like Twitter than Facebook in that their only real monetization path is ads, but their ads don't seem that valuable.
If I'm selling watches, I want to pay Google to advertise to people who just searched for watches. If I'm selling retirement planning, I want to pay Facebook to advertise to middle aged people who like personal finance topics. But who wants to buy Snapchat ads? Some people who want to do brand awareness marketing on a trendy platform but not much else. So I think just because they can get lots of users does not guarantee success.