Facebook grew so much, almost everyone has it and its use cases are versatile (and therefore the data collected). Snapchat is really just used by younger folks with almost no financial power for chatting in the weirdest way.
Even if Snap never gets much traction past people in their 30s, that's enough of a market to expand for quite a while. The 18-34 demographic is the most coveted. The pre-teen and teen market isn't something to completely sneeze at either.
Also it's private. Facebook is mainly used by people with friends and family. Celebrities/notable figures rarely interact with others outside of Twitter. But even with Twitter, the interactions are permanent -- even when tweets are deleted.
On and it's great for sexting, which let's not kid ourselves, was the original appeal and still is for a lot of people in their target demographic.