Dropbox had a good idea, big companies cloned them, and they're still around.
That being said, Snapchat definitely doesn't have some kind of birthright to make heaps of money off the idea. If there are few barriers to entry and they can't out-execute FB, then they will end up losing, and there is nothing wrong with that.
Maybe the discussion should center more on how inherently dangerous it is to build something on the FB platform vs pursuing other opportunities.
This is called "competition". I think what's really telling is that it took facebook less than 2 weeks to roll out.
Meanwhile, when I share anything in my Dropbox, personal or shared, it basically syncs real time - as in, while we are talking on a chat, I drop a file into my shared folder, and seemingly at the same time I say, "I dropped a file in our shared folder" the person at the other end says, "Yup, I got it"
It's beyond me why Apple still can't get photostream to be that responsive with their resources and control over the platforms.
Dropbox had to figure out when a file landed in a OS X folder - Apple controls the operating systems!
Just goes to show that there are no hard rules, everything is case-by-case.