The common theme is to create something truly engaging for your market, at the right time:

Instagram was released at the perfect time when many people had iPhones with a decent camera but limited ways of sharing those photos while on the go.

SnapChat tapped into a similar (though younger) demographic that now needed a more personal space to share, as the mainstream social networks pressed forward with "no one needs privacy anymore".

I'm writing a post covering this exact topic, to be published soon.