This is admittedly anecdotal, but they seem to be driving at least some of their growth through dodgy tactics. Last time I stayed in a Vegas hotel, I went down in the morning and found a Postmates flyer on my car encouraging me to use their service while I was in town. I'm fairly certain that no large casino would give them permission to do this.

If they are doing this kind of thing in cities everywhere, I can see them growing pretty fast, as long as no one sues them or otherwise stops their "growth hacking" plans.