Facebook is an example of a company that continued user growth very well. Facebook was originally just for college students and had a very dating-site-ish feel (at least as I experienced it), but as Facebook grew, they transitioned the platform to accommodate all users from all walks of life, and now their usage numbers are astronomical. I don't see Snapchat doing this very well, their design is very insider-y, their marketing and communications seem to continue to target western millennials, and there's no clear path to adding features or platform dynamics that will bring others in.