These were companies that were almost specifically designed to appeal and be sold to other services like Facebook. A need was identified (sharing pictures and videos with friends instantly) and they were simply first to apply the innovative concept and marketed it better than anyone else.
Common ideals are normally the ability to instantly share media, but usually focus on being at the right place at the right time. For example, Instagram was introduced at a point where taking pictures had reached it's peak, but being able to share them via mobile in a 'Twitter-esque' style feed was non-existent. SnapChat banked on the rise of 'FaceTime' style video chats.
