A killer app is an application compelling enough to win success not only for itself, but also for the platform on which it runs. Examples at Wikipedia and elsewhere include VisiCalc (spreadsheet on the Apple II platform) and Tetris (game on the Game Boy).
One of the great strengths of a platlication is that it can act as its own killer app. For example, Twitter the application attracted users in such quantities that Twitter the platform became attractive to the developers of further applications.
When users invested time and attention in Twitter (the app) they in effect bought Twitter the platform. This is different from Tetris players’ investment in the Game Boy platform: a Twitter user invested no money, and was not conscious of any investment in a platform. But the similarities are more important: developers knew that there was a large potential audience for apps on the platform.