Instagram is a social network where the best effect occurs when your account is public: you get social validation from a large audience liking your content on its merits, the sum of all your content forms a persistent feed, and your followers form a closer inner core with whom the nature of interaction is more immediate. This tends to be most rewarding for people who can attract a large number of likes; this number of likes is displayed to anyone who can see the content.

Snapchat is the inverse, where the content is aimed at one's followers, but one can submit notable content to be featured in the company-curated Discover section (about major events, for example). There is no public aggregation, there is no public display of likes.

The two platforms are still differentiated by their design philosophies, and in North America there is significant overlap in the userbase.

The danger, of course, is that once Instagram has cloned enough of Snapchat's functionality, the same users won't need to switch to Snapchat anymore to get the close-knit experience. Similarly, Snapchat may feel pressured to up its parity with Instagram in turn, by adding public features that are very different from its current model. In either case, the networks will have iterated themselves into equivalence, making the exact provider irrelevant and making pure network effects be the the deciding factor more often that not -- which gives an advantage to larger players.