What would give one company the advantage in a monopolistic competition market w/o product differentiation?

I was just talking with a friend about how stupid it was that Snapchat let the other person know if you were lurking around the chat, and it got me thinking that if say, Snapchat, was functionally identical to Instagram and they were both in the market of social media apps, what would be the thing that sets either one of them apart or put them in a better position? Timing?