The key difference is that all those other services you list have UX that is mostly centred around active use ("Check this out: Your friend Joe is doing a thing right now!"), while Instagram's UX is mostly centred around passive use ("Now that you have some free time, let's catch up on what your friend Joe did last week").

Ultimately they can all be used for the same purpose, sure. At the end of the day you're pushing the same bits around. But UX can make all the difference.