They weren't a Snapchat killer but they immediately cut off future growth and stalled it out for a solid 2-3 years.

You don't have to kill the competition in social to win, you just need to stop their growth. They can retain some percentage of users but if the growth is stalled out and you are dominate them in 5:1 marketshare it's as good as dead.

Snapchat is interesting - in my age group (mid to late 20s), it got obliterated by Instagram a few years ago, to the point where it feels "old" to use it. However it is very popular with younger people (college and early 20s).