  Online social networks have increasing influence on our society, they may
play decisive roles in politics and can be crucial for the fate of companies.
Such services compete with each other and some may even break down rapidly.
Using social network datasets we show the main factors leading to such a
dramatic collapse. At early stage mostly the loosely bound users disappear,
later collective effects play the main role leading to cascading failures. We
present a theory based on a generalised threshold model to explain the findings
and show how the collapse time can be estimated in advance using the dynamics
of the churning users. Our results shed light to possible mechanisms of
instabilities in other competing social processes.
